<iframe src="https://player.vimeo.com/video/270545015?background=1&quot; width="640" height="360" frameborder="0" allowfullscreen></iframe>

Hang on to a pull-up bar and place a dumbbell or some sort of weight between your feet, (or use no weight for an easier modification.) Lift knees up to your chest then back down, using your core to bring the weight/your knees up and down.