1. Set up in a plank position on the elbows with your feet on sliders. Don't sag.
2. From here, it works like a reverse ab rollout. Pull your body back and forward.
3. If you catch yourself sagging, you've gone back too far for your abilities.

Not only is the body saw a great core exercise, it can also train the serratus, an important scapular stabilizer. Just add a band around your wrists.