Lie on your back with your knees bent and your hips flexed into your stomach.
With your elbows slightly bent, hold on to the bench just behind and above your head. If you're on the floor, hold on to a solid object above your head.
Do a reverse crunch by rolling your lower back up off the floor and bringing your knees toward your chin. Don't use momentum.
Slowly reverse this motion, lowering your spine back down one vertebra at a time. Don't allow your legs to extend or your head to lift off the surface at any point.