Rotating Plank

Can be performed on your forearms or on your hands: "Stack" your forearms, touching them together (as opposed to parallel), and, shifting your feet to their sides, one behind the other, bring one arm laterally out to the side, and, using your obliques, lift the arm up into the air. If it helps, you can turn your head to look up at the ceiling. You want your chest facing out to the side completely instead of it remaining facing the ground.

If you'd rather do the exercise on your hands, start off in push up/plank position. Watch the feet in the video to understand the rotation.