Position yourself for this exercise by looping a medium-sized towel around a pullup bar and firmly grasping it on both ends. Hang from the towel with your arms and body in a fully extended position. Bring your torso back at about 30 degrees and create a curvature in your lower back by sticking out your chest. Pull your torso upwards until the bar touches (or almost touches) your upper chest. Focus on keeping your upper torso stationary. Once you have lifted your entire body up by the bar, pause briefly (squeezing your back, bicep, and forearm muscles) before slowly lowering your torso back to the starting position until your arms are fully extended once again. Repeat for the prescribed number of repetitions.
Once you can perform this exercise successfully for very high repetitions, try adding weight via a weight belt to make the motion more challenging.