Want to Know What a Core-quake Feels Like? Try Jordan Chiles's and Simone Biles's Ab Workout

• Lie on your side with your arms and legs extended. Bring your feet slightly in front of you so you're in a hollow position. You can put one hand down for support if you need (as seen here).
• While keeping your navel pulled into your spine, simultaneously lift your arm(s) and legs a few inches toward the ceiling. Engage your core as you hold this position, then slowly rock from front to back, imagining that your body is a seesaw.
• One front-and-back motion equals one rep. Do 10 rocks.

Complete all three of these side hollow moves on each side.