A great alternative to the regular crunch or sit up as it reduces a lot of the shear stresses on the spine.
Lie on your back, bring your knees over your hips and tuck your heels into your butt. Place your arms by your sides, palms face down.
Keeping your arms flat on the ground, think about peeling your spine off the floor from the tailbone up. Keep your feet tucked as the knees come towards the chest.
Adjust range of movement to fitness level. Beginners may find it easier to hold something with their hands above their head.