This exercise is used to work the muscles of your back, which includes the lats and the middle of your back. This movement will also work your biceps more than the same movement with a wider grip.
Reach up and grasp a pull up bar with your palms facing away from you. Your grip should be a little tighter than shoulder width. Begin by hanging from the bar. Next, pull your body up until your chin rises above the bar. You should slightly lean back and look up, this will force you to use the bigger muscles of your back. Once you are at the top, hold for a second and squeeze the muscles of your back. Next, slowly lower yourself back to the starting position, but make sure not to allow your feet touch the ground. This will keep constant stain on the muscles of your back and forearms.
Pull ups are much harder than doing the same movement on a lat pull down machine. Since, you are hanging freely your body will to stabilize the movement. Many people can do a lot of weight on a pull down machine, but many people can’t do a full set of pull ups.