Start in the dead-hang pull-up position. Instead of pulling straight up, pull to your left side, touching the top of your left pectoral to the bar. Descend under control, then do the same thing to the right.
You can also go side-to-side for two reps before descending: pull to the left, hold at the top, transition to the right, then lower yourself. Start the next rep by pulling to your right.
Doing this glute-builder in a rack acts as a depth gauge, much like box squats. It keeps your form in check and it makes it easy load and unload plates.