Set-up: Find a long towel, rope or an otherwise suitably strong material. Brace through your core and establish a neutral spine. Grasp the towel with one hand at each end, and fix the bottom underneath the middle of one of your feet.
Movement: Curl your arms up to your chest, keeping your elbows at your side and eyes straight ahead. Use your leg to press against the towel and provide appropriate tension throughout the range of motion of the exercise. Do not let your upper back round over - consciously pull your shoulder blades together to provide a stable base for your arms to pull from. It may take a few practice reps, or even sets, to get the feel of this exercise.
Emphasis: Do not swing - keep your core tight and a neutral spine.