HOW: Start in the tall plank position. 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 blades. 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 push up plus, not a full push up.