High Intensity Interval Training (HIIT)—which packs as much intense energy as possible into as little time as possible—is having a serious moment in the fitness spotlight. While HIIT has been used by high-performance athletes and trainers for years, it’s showing up in more gyms and fitness studios than ever.

The evidence? Jillian Michael’s new BodyShred classes at Crunch are based on her 3-2-1 take on HIIT. Equinox’s Tabata, Metcon3, Whipped, and 30/60/90 are all HIIT variations. Central Park’s newest bootcamp is HIIT-based, as is Brooklyn-based Beastanetics. Celeb trainer Amanda Russell just launched a 14-day fitness program with HIIT workouts.

Not to mention the CrossFit boom, which isn’t exactly HIIT, but shares lots of similarities.

So what is it? Basically, it’s a series of very short, super-intense, cardio-strength intervals (think burpees or jump squats as fast as you can go), with short recovery periods in between, all confined to a time period that’s usually between 10 and 45 minutes.

Why has the favored fitness regime of athletes become a popular workout for the rest of us?

Recent studies have shown that short HIIT sessions can be more effective at improving heart health and building muscle than much longer moderate-intensity workouts. And New York Times reporter Gretchen Reynold’s best-selling book on current exercise science, The First 20 Minutes, is chock full of facts on its benefits.

It’s drawing people because “it’s efficient and effective,” says People’s Bootcamp founder Adam Rosante. “One short HIIT session will torch fat, build long, lean muscle, and boost your metabolism more than any other workout. I’ve seen clients experience visibly noticeable weight loss and impressive strength gain after a little over a week.”

Carol Espel, Equinox’s senior national director of group fitness and Pilates (who also has a master’s degree in exercise physiology), agrees. “We all know you get the most work done, the most value in calorie burn and aerobic fitness. So people are saying ‘I don’t have to do anything else. I’ve gotten everything here.’”

And efficiency, in New York, is king. What busy executive doesn’t want to be able to fit a full-body workout into a lunch hour and still have time to eat?

Plus, when you’re doing HIIT, your body still continues to burn fat and calories when you’re back at your desk, and for several hours more, say some experts.

That afterburn effect comes from EPOC (excess post-exercise oxygen consumption), which requires energy to help restore the body to a resting state and adapt it to the exercise just performed.

One more aspect that makes HIIT appealing? What it does for fitness-lovers’ minds, not just their bodies, says Espel. It’s about breaking mental barriers. “It’s the kind of thing where you push yourself in ways you never thought you could,” she says. “For some people, that’s all it is—getting through it.”

Of course, not everyone wants to be pushed to their edge every time they workout. But for Type-A time-pressed individuals, HIIT may be a golden ticket.

“If you really want to see some results, you have to get uncomfortable,” says Amanda Russell. “You have to throw your limits out the window.” —Lisa Elaine Held

Have you tried High Intensity Interval Training? Tell us in the Comments, below!