Besides showing off to the ladies, these widen each side of the back, independently from the other. They take tremendous, skill, balance and coordination, on top of muscular strength. You might master them on an assisted pull-up machine to start or use mixed chins to build coordination and strength. The width you receive from these makes them well worth the effort!

Implementation: (A) Place one hand(palms facing you ) on a pull-up bar and the free hand on a rope or towel that you have hung from the bar (for support). I simply rap my towel around the bar and grasp it. Start by grasping the rope up high for your pull-ups and as you continue to get stronger, you'll be able to place your hand lower and lower on it.
Now with your full concentration on the arm grasping the bar, perform your pull-ups as strictly as possible. When your finished, repeat with opposite arm.