I'm going to demonstrate how to do a body saw with a foam roller for you. So you could use a small foam roller like this. If you use a larger one, you're just going to place your shins on the center of it. The small one is more convenient because there's not a lot of room for error. So what you're going to do is you're going to get into a normal plank position. I like to place the foam roller down by my feet already. I usually put it by my shins, and that's what your going to do. We're going to make sure our alignment is right, putting our elbows right under our shoulders.
You're going to lift your knees. Now the foam roller should be right on your shins. It should be on the lower part of your shins. What you're going to do here is you're just going to simply shift your body weight back and forth like a saw. You're going to do this either for 30 seconds... You can either do it for seconds or you can do it for 15, 30 reps. It's one of my favorite ab exercises and it definitely targets the lower abdominal wall. And that's how you do a body saw with a foam roller.