Works your obliques and shoulders and builds core strength and stabilization.
In a forearm side plank position (A), engage your abs and reach your top hand toward the ceiling (B). Slowly tuck your left arm under your body and twist forward until your torso is almost parallel to the floor (C). Return to side plank. Repeat 6-10 times then switch sides.
Make it harder: Go up onto your hand with the standing arm.