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 lift one arm and tap your opposite shoulder. Repeat on the other side.
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 when picking up your opposite hand. Keep strong through the shoulder the entire time. If it is too hard for your core, widen your feet.