Feel the burn while barely moving with this abs exercise from Chelsea Handler’s trainer

Reps of jump squats and burpees definitely pose the potential of leaving you sore for days, but sometimes it’s the exercises that barely require you to move at all that result in the most serious body burn. Just ask Chelsea Handler, whose trainer recently had her do his favorite abs workout at home. Spoiler: It’s way harder than it looks.

Ben Bruno—who also trains Kate Upton—recently posted a video of Handler doing one of his go-to core exercises: hollow-body flies, which requires you to hold a crunch with elevated legs while simultaneously lifting and lowering a dumbbell. They’re so intense, in fact, that Handler tried to negotiate the number she had to do. “I told her to do four sets, but she insisted we only do three sets, so we compromised and did five sets,” he writes.

Bruno loves the super-challenging core exercise for a good reason: It fires up your abs like crazy from every angle. To try them yourself, lay on your back, lift your upper body into a crunch position and hold, then slowly lift your legs, keeping them straight and close to the floor. While holding one arm straight up in the air, lift and lower the other arm holding the dumbbell—and make sure to keep the weight light. “The straighter you keep your arm, the harder it is for the core,” Bruno writes. “I actually don’t like doing heavy flies for the chest because I don’t think they’re great for the shoulders, but in this case, the weight is so light that I’m not worried about it.”

According to Bruno, your core will be on fire even if you do the movement without any weight, so when you’re first starting, he says to only stick to a few pounds at most. By the time you’re finished with all your reps, you’ll have a sense of why Chandler was negotiating to try and do fewer—but you’ll no doubt feel proud of your burn-building accomplishment.

Here’s how Chelsea Handler boosted her metabolism without cutting out alcohol. Or, check out her spot-on reason for choosing a career-focused life instead of one that involves having kids.