This is a chin-up but instead of focusing your strength on the 'up' part of the movement you'll be focusing on the 'down' part of the movement-the negative. Position yourself under a chin-up bar with your hands above your head, wider than shoulder-width apart.
Jump upwards and grab the bar in a wide-grip so that you are in the final 'up' position of the chin-up movement. Try to use your legs to launch yourself up into this position as much as you can. You don't want your back to be doing all the work.
Once you're in the full up position, pause for a one-count and slowing begin your descent. You should take a full five-count completing the negative movement. Once down, reposition yourself, launch yourself upwards and repeat. As it gets more difficult to launch into the up position of the chin-up, position pegs to use as steps to assist in getting yourself into the starting position.