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.
Tony shoots apart training myths like they were ducks and he was a starving fat man sitting in a pond with a rifle. Read about the "other side of the core," deadlifting mistakes, and the pencil test. (You gotta' take the pencil test.)