Begin in a side plank position with your upper body propped up on your right elbow and forearm. Place your left hand behind your head. Brace your core by contracting your abs forcefully as if you were about to be punched in the gut.
Point your top (left) foot and lift your leg slightly higher than hip level. Trace a 'rainbow' with your upper leg by tapping your foot to the floor in front of your bottom leg, bringing it back above your hip, and then tapping it to the floor slightly behind your bottom leg. Repeat 15 times in a row on each side.