The bodysaw is very similar to the ab wheel rollout. The goal of both exercises is to resist extension of the lumbar spine (avoid arching your back too much).
To do the bodysaw, start by getting in a plank position with your feet on something slippery such as Valslides, a slideboard, furniture sliders, a paper plate, a TRX, etc. From there, maintain that body position and push back and forth on your arms, like this:
Go back only as far as you can handle while still maintaining your original spine position. If you start to arch excessively and/or feel them in your lower back, you've gone too far. They're a lot tougher than they look, so it probably won't take much range of motion to feel them working.
Once you've got that down and it feels easy, you can progress to doing them on one leg at a time or, if you want to get really frisky, doing them with straight arms starting from the bottom of a push-up position, which extends the lever arm and makes them pretty brutal.
Don't jump into this version too fast, though, because you don't want to hurt yourself.
The bodysaw has a few advantages over the ab wheel rollout:
- Most people feel them more in their abs.
- Bodysaws don't fatigue the shoulders like rollouts can, so they're easier to pair with upper body exercises.
- They're more user-friendly for people with preexisting shoulder injuries and/or poor shoulder mobility. They're easier on the lower back too.