Begin in a high plank position with your left hand next to a dumbbell. (Your hands should be slightly in front of the weight.) Maintaining a stable core, keep your hips still but allow your shoulders to twist as you lift your right hand off the floor and reach under to grab the handle of the dumbbell. Keeping solid contact with the floor with your left hand, press down even harder while you drag the dumbbell underneath you from left to right with your right hand, ending with the weight outside your right shoulder. Plant your right hand on the ground, and begin the movement again in the opposite direction with your left hand.