Note: Have patience with this one. Find the limits of your comfortable range of movement and don't seek to extend this range too much, too quickly.
Using paper plates, gliders, or a towel underneath your feet, get into Day 1's Forearm Plank.
Holding a solid plank, pull your whole body forward so shoulders move closer to hands. Keep hips in line with with head and heels. The body sawing action is always slow and controlled in both directions.
Keeping those corset muscles engaged, and hips in line, push the shoulders back until they are over the shoulders again.
The tough work really begins once the shoulders move back past the elbows.
Only go back as far as you can control and return back to the starting position. Each time, try to extend your range of movement but stop short of that point where your corset muscles fail and the lower back gets pinched.
Basic forearm plank, 1 min total time.
1/2 Body Saw: only pull shoulders to hands and back to the starting position over the elbows.
Reorat 10-20 times
Same as Level 2, but for 20-30reps
Full saw range of motion x 20 reps.
Full saw range of mvement x 40 slow reps.