This is called a Stability Ball Dolphin. You are going to position yourself over the ball with the top of your hips on top of the ball. Your elbows are bent on the floor and you are going to lift your feet straight up in the air squeezing your glutes and coming straight back down. You want to make sure to keep your core nice and tight to keep from rolling off the ball. Also, you want to make sure that your feet stays together the whole time.