Ab wheel rollouts are awesome for building core strength and stability, mastering the hollow-body position, and learning how to create and maintain the total-body tension that will take your pullups, deadlifts, and squats to the next level. The only problem is that many guys butcher the move when they try to add it to their workout, says Jeff Cavaliere, C.S.C.S., a physical therapist and strength coach who you might follow on social media for his Athlean-X training program.

The most common error Cavaliere sees is when guys bend at the hips as they finish off each rep. After all, your hips and glutes are part of your core. When you bend, you break your total-body tension. You also take some of the onus off of your rectus abdominis (six-pack) muscles, meaning you get far fewer core-strengthening benefits from each rep.

Check out Cavaliere’s smart demo series to help you nail your form. His first three reps are the way you want to perform the rollout—his hips are stable and controlled. He sets up on his knees, and then he maintains that strong position throughout the entire exercise. He doesn’t bend or sink his butt back toward his heels at all.

If his instruction doesn’t make sense, keep watching the next three reps. Here, we see Cavaliere showing what you should avoid at all costs. See his hips bend and his butt almost touching his heels at the end of each rep? That’s how most guys get it wrong—and it’s exactly what you won’t do any more.