The rope is purple. It stretches long and low between two palm trees, two of many that line the grassy portion of La Jolla Shores. It is attached to said trees — their trunks carefully padded — via a series of cables and caribiners (oval-shaped metal rings that open and snap closed), and sways peacefully in the ­breeze.

Not far away, Frankie Nagara sits cross-legged, his back to one of the trees. ­He’s a lightly mustached 23-year-old, a sharp-ended plug adorning each earlobe. Soft-spoken and relaxed, he is clearly about to say something, but before he can construct his forthcoming sentence, an older woman stops on the sidewalk and ­balks.

­“What’s this rope for?” she asks, her voice strident, as though she expects she will not be able to hear the ­answer.

“Slacklining,” Frankie replies, not missing a ­beat.

“What ­lining?”

“Slacklining.”

“Slacklining?” The woman echoes, hitting a new octave entirely. “What is ­that?”

“This piece of rope right here,” Frankie patiently explains. “You walk on ­it.”

The ­woman’s face lights ­up.

“Kind of like tightrope,” she ­says.

­“It’s kind of like tightrope, yeah,” Frankie concedes, “but ­it’s a lot looser. You should give it a ­shot.”

The woman smiles politely by way of declining, pausing a moment longer before continuing on down the sidewalk. Frankie shrugs as if to say, her loss.

The rope ­sways.

Slacklining, a practice generally known as “slacking,” looks deceptively easy. Those who are good at it walk across the rope with the grace of gymnasts; those who are great at it can jump and flip, landing back on the line, or balancing themselves in yoga-like poses, faces drawn in ­concentration.

Beginning, however, takes considerable determination. The line wobbles uncontrollably, as though ­one’s body has a constant and powerful tremor. Walking it with the aid of another person, who acts as a helpful counter-balance, feels not unlike what traveling across a large rubber band might; the line is at once springy and taut, reacting to any change in weight ­distribution.

This, done successfully or unsuccessfully, is called “walking the ­line.”

­It’s damned ­difficult.

Frankie, however, is a master. Step by step, he balances in the middle of the rope, which is one inch in width and relatively flat, and then proceeds to curl himself into a pretzel shape, bringing his hands into a prayer-like position, palms ­together.

This is called the Garland ­Pose.

Frankie, who is the founder of sandiegoslacklining.com, and the San Diego Slackline Meetup on the popular in-person social networking site meetup.com, has been slacklining for nine months. He started when his fiancée, Megan, who balances on a second line they have set up, began to encourage ­him.

“She kind of shot the bug out,” he recalls. “[I said], ‘Hey, I want to try this,’ and [she said], ‘Definitely do it! I want to do it, too!’ Then, seeing pictures of friends and other people doing it, made me want to do it ­more.”

Megan smiles and says softly, “I did it with a couple of friends, some climbing friends, a few years ­ago.”

While the particulars of ­slacklining’s origins are hazy, what slackline practitioners (often called simply “slackers”) agree on is that the practice began in Yosemite, California, in the ’70s and ’80s and was born from the widespread rock-climbing community there. Climbers, many slackliners say, started unofficially slacklining while waiting at the base of rock faces or back at their camps, stringing up yards of webbing and working on their balance. Most slacklining gear is (or was) intended for rock climbing and has since been ­appropriated.

As far as sports go, slacklining is inexpensive. Unlike surfing, where one must shell out anywhere from $400 to $1000 for a board, the webbing used for slacklines is cheap, cheap, cheap. At popular outdoor retailer REI, the one-inch tubular variety starts at .36 cents a foot. Caribiners are anywhere from $9 to $24.

Slackline-specific gear is a bit more costly. Slackline Brothers in Los Angeles sells ­what’s called a “slackline starter kit,” which includes 75 feet of tubular webbing, and the “Slackline Brothers Tightening System,” for $230. Gibbon Slacklines, a company operating out of Germany (more on them in a moment), sells their ratchet-system slacklines starting at $79.99. But this pre-packaged gear is by no means necessary; slacklining started as a do-it-yourself sport and the tradition ­continues.

And ­it’s catching on. There are talks of competitions, one of which was already held in Slovenia, and, Frankie says, groups worldwide are discussing rules and regulations for official slackline ­games.

In San Diego, ­it’s mostly done for ­fun.

“With our actual group, San Diego Slacklining, we stand at about 50 members,” says Frankie. “But ­there’s definitely a lot more people that slackline in San Diego that just ­haven’t signed up for our site or for our group. [There are] probably upwards of 100, 200 ­people.”

The San Diego Slacklining group, which Frankie started in April of this year, typically draws a crowd anytime they get together and ­slack.

For the two days following the La Jolla Shores set-up — Fourth of July weekend, no less — Frankie, Megan, and company meet at Moonlight Beach in Encinitas, along with a team of guys from Gibbon Slacklines, the aforementioned German company. The Gibbon crew is in the midst of a two-month American tour, and San Diego happens to be a stop on the way; having hooked up with Frankie, they are privy to some of the best slacking spots in the ­city.

Moonlight Beach is packed. Brightly colored umbrellas create a psychedelic landscape, and a band plays upbeat cover songs under a Belly Up Tavern banner to a large crowd of swaying children and adults. Bare skin abounds, tattoos flashing as people flit past dragging boogie boards and toddlers behind ­them.

The assembled group of slackers, a motley crew of men and women, Germans and Americans, begin to set up their lines. Instead of being tethered to trees, the lines are attached to “A-frames,” two-by-four-and-plywood constructions that resemble a large “A,” with metal rings on each. One of the A-frames, for extra stability, is tethered to a fencepost, while the other serves as a middle point for a second line strung to a concrete picnic bench. Between the two lines, the A-frame is effectively pulled in two directions, rendering it ­stable.

The German system is simpler. Strung between two fence posts, and then, later on, two pipes sunk into the asphalt parking lot, their lines attach simply and are tightened via a ­ratchet.

Setting up the Gibbon lines is Hogart, a tanned, shaggy-haired engineer from Stuttgart, Germany. He started slacklining in August of last year, a self-described ­“young ’un,” a colloquialism that sounds charmingly bizarre in his accented ­English.

“The first two hours [were the hardest],” he says, of his slacklining inauguration. “At the beginning, you see [people slacklining], and you think, ‘Oh, it looks so easy.’ Then you try it and you think, ‘Oh, damn! ­I’m shaking! ­I’m not fit, ­I’m not good!’ You have to get over this point. You have to believe that everyone is shaking at the beginning, and you have to forget your [pride] and just try it. After that, everything comes on its own. You keep trying, you discover new things, and then you slowly discover your own style. Because everyone has their own ­style.”

What draws Hogart to slacklining, he says, is in part the allure and thrill of doing tricks, but ­it’s also because he finds it to be intensely ­relaxing.

“You walk the line and focus on yourself and what you are doing,” he says. “Because it takes so much attention, your mind is clear. ­It’s very good for stress relief. When ­I’m really stressed from my work, then I just take my slackline and go into the forest, plug in my music — Celtic-Irish folk or something — and within half an hour, ­I’m done with my stress. ­It’s ­gone.”

Along with the Gibbon team is Mike, a 19-year-old from Boulder, Colorado, who answered ­Gibbon’s call for a summer tour mate from the United States and landed the job. He is the one who spies the two metal poles. He removes a cordon chain and replaces it with ­Gibbon’s two-inch ­slackline.

­“We’ve been here about two days,” he says, brushing hair from his eyes. ­“We’ve been to a lot of places, and generally, we have to find places [to slack] on our own, which is a little more difficult. But since ­we’ve been hanging out with [Frankie], ­he’s shown us all the best spots, all the ­slackliner’s secrets, and ­that’s gotten me a greater impression of San Diego. ­It’s a great town. I love ­it.”

Mike started slacklining during his first year of college, about a year and a half ago, where the resident advisor in his dorm had set up a line. He was hooked instantly and took it up on his own. Since it is now illegal in Boulder, Mike must slackline on the sly when in his hometown, but, touring with Gibbon, he can do so ­freely.

Though San Diego has no official stance on slacklining, the crew was asked to take down their lines at Mission Beach on July 4, citing a municipal code Frankie took down on his phone, SDMC 63.012(b)(4): “Use of Parks and Beaches Regulated, Destruction of Plants.” It states, “It is unlawful to injure, destroy, cut or remove any tree, shrub, plant, wood, turf, grass, soil, or rock in or growing in any City-owned park or plaza without the written permission of the City ­Manager.”

But, at Moonlight Beach, there seems to be no problem. A police car marked “Sheriff” is parked not far off, and a few cops pass by without a second ­glance.

A crowd has gathered at the lines, mostly younger children. They take bold steps on the lines, managing to balance for a few moments before falling onto the ­sand.

There are several types of lines, Mike says, chiefly “high lines,” which are lines strung up 50 feet and higher, walked with a harness for safety; “long lines,” which are any length outside ­one’s skill set and are mainly for walking; and regular old slacklines, the kind the group has set up on the beach. These lines, typically three to four feet off the ground and a few yards in length, are the best for doing tricks and for beginners to get the ­feel.

This is what Maggie, a member of the American contingent, is doing, getting the feel. Though she has injured an already-hurt knee, it does not deter her. This is, she says, her fifth or sixth time at a slacklining ­event.

“I think ­it’s good for me,” she says, eyes shielded from the brightness of the day by a hat and a pair of sunglasses. “Spiritually, ­it’s good for you because ­you’re totally in tune with the line and where your brain is and how your body works. And physically, I think ­it’s good because it builds your core. When ­you’ve had a good day slacklining and are really sore the next day, ­you’re working muscles, even though it ­doesn’t seem obvious at the ­time.”

Despite her knee mishap, Maggie views ­slacklining’s physical risks as ­few.

“I think ­it’s probably safer than swimming the ocean or surfing,” she says. ­“It’s like any sport when you first start. There [are] lots of attempts before you get good at it. And so you pop off the line, which is why you pick a nice, soft spot to land. But, you know, ­I’ve had the line snap and hit my skin, and it stings a little bit, but ­it’s not a big deal. Overall, I think ­it’s pretty ­safe.”

None of the slackliners interviewed have sustained any major injuries. Both Megan and Frankie bear what they call the mark of a slacker, a half-inch-long line of grazed skin that runs the length of their shins. Hogart says he has “punched his family jewels,” but other than that, has not harmed himself slacklining. Ditto ­Mike.

As the day wears on, the newbies, mostly a gang of 12- to 14-year-old skater boys, gain confidence on the lines, and in the expert camp, the tricks begin. Mike and Hogart take turns on the Gibbon line ­they’ve strung above the asphalt — an added risk — jumping and ­leaping.

Mike, cheeks puffed out, bends his knees, line beginning to wobble precariously. This he has anticipated. In a smooth motion, he launches into the air, turning in a full 180 rotation — and lands ­it.

An onlooker, a kid maybe four-and-half-feet tall, hair bleached blond at the tips, stares, eyes ­agog.

“Cool,” he says. ■