If you had childhood dreams of running off and joining the circus, the slackline fitness trend is a healthy way to revive your lost passion and get an incredible workout in, too. Gunnar Peterson, who’s trained celebrities including Khloe Kardashian, Amber Heard, and Jennifer Lopez, posted a video on Instagram of the technique in action, and it’s equal parts fun and brutal.

Slackline fitness—also known as slacklining—involves performing exercises on a narrow, flat line of webbing that’s suspended between two anchors low to the ground, either on a workout machine or between to trees or posts, old school-style. It’s similar to tightrope walking except that the line is, as the name implies, slack instead of tight. This makes it especially difficult to maintain balance.

Peterson showed off fellow fitness pro Paige Hathawaytesting out her skills on a machine at his LA gym (to the tune of Kanye West’s “Fade,” for additional enjoyment).

“There are many benefits to slackline training including improving your balance, core, and building lower body strength,” Peterson wrote in the caption. “Training should never be boring…It should be fun and challenging! Get out there and switch up your routine!” Peterson’s post shows Hathaway doing step-ups with a reverse lunge, a compound exercise on its own that targets your glutes, hamstrings, quads, and abs, using the slackline machine. As you can see, the slackline adds a major challenge.

The wobbly training tool requires laser focus and a willingness to get back up there when you stumble (which definitely happens). Although Gunnar uses the slackline as a strength-training tool, most enthusiasts start off by trying to stay on the slackline as long as possible. You can then progress to walking, crawling, or even performing tricks such as handstands or lunges on the slackline. To see outdoor slacklining in action, check out Swedish athlete Jenny “Kitsune” Adolfsson practicing a basic slackline walk:

Slacklining is particularly incredible core challenge, says fitness and lifestyle expert Lisa Tanker, C.P.T. “Your core comes into play with anything that requires balance, because you have to engage your core to stay stable,” she tells SELF. But even though your core is the major stabilizer here, a balance challenge this intense engages pretty much your entire body, including your ankles, calves, thighs, and hips.

Working on your balance is an important part of a fitness routine in its own right, too—better balance helps protect you against preventable injuries from stumbles and falls, says Tanker. (If you self-identify as clumsy, working on balance is a must.)

That said, it’s important to be safe with slacklining, because it can cause injuries if you’re not cautious. This isn’t exactly a beginner workout tool, so Tanker recommends starting slow. For example, don’t jump straight into exercises like Hathaway’s step-ups with reverse lunges if you haven’t mastered the balance element on solid ground. If you’ve never slacklined before, begin by just trying to balance on it. It’s best to do it with other people around to spot you (and a trainer, if you’re doing actual exercises) and set the line up above a soft surface, like grass or sand. And as always, if you have any injuries, make sure you check in with your doctor before starting a new activity.

If you do decide to channel your inner acrobat with a slackline, here are a couple of other examples of the tool in action. Olympic shooter Sarah Scherer uses a slackline machine for balance training:

You can also anchor a regular slackline to a couple of trees for a fun outdoor workout. Tanker notes that this is one of the best parts about slacklining: Even if you’re not doing lunges in a gym on a machine, it can be a fun way to try something different with a group of friends.

If you’re ready to give it a go, you can buy slacklines online. Again, safety first: Follow the instructions for your individual slackline so you know how to make sure it’s secure, how far it should be from the ground, and how to get started.

“It may not be your full workout, but it’s a way to test yourself and try something new,” says Tanker. Happy slacklining!