Lie faceup with your knees bent into your chest, arms on guard. Lift your head, shoulders, and upper back off the ground and then perform a bicycle crunch by twisting your left shoulder toward your right knee, left leg extends out straight and parallel to (but not touching) the ground. Repeat to the other side. That's one rep.
Do 20 reps in a row (head stays lifted the entire time) as quickly as you can. Keep your belly button pulled in tight to your spine and your chin in towards your chest to maximize your belly burn and avoid neck strain.