To perform a band resisted pull-up, take the pins from a squat rack and drop them down the lowest setting. Take your resistance band and loop it around both pins at the floor. The band should be running parallel to the floor at this point. You can do the next step one of two ways. You can either wear a weight lifting belt and run the band through the belt and then put it on tightly. If you don't have a weightlifting belt you can also use a dip belt and run the band through the chain. With the band attached to your belt, reach up and grab the pull-up bar on the rack or use the frame of the rack as your pull-up bar.
This exercise is very tough and should be only be used by advanced lifters. For the really freaky strong, you can use both parts of the band and run them through the belt. You can use either a pronated or supinated grip.
Band Resisted Pullups Back Exercise - Start Position
Band Resisted Pullups Back Exercise - Finish Position