This works your triceps, biceps, core, and waist.
Lie on your right side with forearm directly under shoulder, hand perpendicular to your body, and legs stacked. Engage your abs and the right side of your waist, lifting your hips so your body forms a straight line from head to feet.
Extend your left arm toward the sky, staying engaged through your core (shown). Now scoop your left arm in front of your body and reach under the space between your chest and the ground, twisting only from the waist up.
Come back up; repeat 4 times, then lower body to the ground. Repeat on the opposite side.