Think of this exercise as a push-up that's mostly for your shoulders instead of your chest. It forces the shoulders to work hard to drive your body back using a natural arc and push angle.
The Push-Back Push-Up
Level 1: Drop down, explode straight back, and return to push-up position.
Level 2: Same basic movement, but put your feet up on a braced bench. Your abs will be screaming.
There are plenty of ways to ratchet up the intensity of a set, like forced reps, Tom Platz iso-holds, and rest/pause sets, but you need to apply them in two-week waves.