HOW: Start in the modified tall plank position on your knees with your hands on the ground. Drive your hands into the ground and also imaging driving your elbows into the ground. This is your stable base position. Keeping the core and shoulders engaged and strong, slowly let your shoulder blades come together. Then drive your hands into the ground again and separate your shoulder blades.
FEEL: You should feel all the muscles in your shoulder working, especially in the front part of your shoulder and under your shoulder bladers. You will also feel your core working.
COMPENSATION: Do not lose your serratus engagement. Keep strong through the shoulder the entire time. Also do not let the elbows bend. This is a modified push up plus, not a modified push up.