Fold forward and place your hands on either side of your legs, somewhere between knee-level and foot-level. The further away your hands are, the more difficult the exercise becomes.
Push on your hands and lift one leg off the ground, up and down without touching the ground. You can do this for time or for repetitions.
Repeat on the other side.
Form tips
Do not lean back: you need to be leaning forward and to push on your hands, keeping your torso as low as possible, to work your compression strength.