The side-to-side circle motion of this move will have your abs contracting in all directions—plus this plie position means your legs will be working hard too!
How to do it: Start standing "on guard" and then step both feet wider than hip width apart, turning your toes out slightly. Bend both knees directly out over your toes, and lower your body straight down, keeping your shoulders over your hips, your back straight, and your abs pulled in. Holding this deep plie position with your legs, lean your upper body to the left, pressing your left shoulder down towards your left hip, keeping your arms up on guard and chest facing forward. Next, round your back and shoulders forward, scooping your abs in deeper towards your spine, and hips steady. Then come up and around to the right side, leaning your upper body to the right. Finish the move by bringing your upper body back to the middle, and straightening your spine, before repeating in the other direction. Do this 20 times in total, reversing sides each time.
Bonus tip: Imagine you are tracing a circle with your upper body, and make sure to create a full circle on both sides with every rep.