Start in a normal push-up position, then allow your chest to move towards the ground by pulling your shoulder blades together
Once at the bottom position, drive the base of your palms into the ground to push your body up and slightly backwards, which will help your shoulder blades upwardly rotate as you go
Push the middle of your back as high in the air as you can with the shoulders above a 90 degree angle (the serratus anterior muscle we're trying to hit here works best above a 90 degree angle