Instructions
Standing on a box or sturdy bench under a pull-up bar
Use a slight jump to get yourself into the starting position with your chin at about the same height as the pull-up bar and your hands in an overhand grip about twice your shoulder width apart.
Bend your knees and bring the heels of your feet up towards your glutes.
You should be now supporting your weight with your chin at the same level as the bar.
When ready, slowly lower your body down towards the box or bench, using your back and arms to control the speed, until you can place your feet firmly at the bottom of the motion.