STARTING POSITION:
Lie down facing the floor and rest your forehead on your hands. You need a partner who grab your legs from your ankles.
MOVEMENT:
In a strong movement, fully flex your knees while you exhale. Your partner should resist the movement. Return to the starting position in a smooth movement while you inhale.
TIPS: While your are doing the exercise, move only the legs, make sure you don't move any other part of your body. The exercise key is quality over quantity. The trick is to perform the exercise using proper form.