In reply to 00spaw: i can lock off but not for long, ive been climbing 8 months and alot of pull ups mixed with lowered rope off the chin up bar and training one armers has helped alot.
holding all your weight on one arm and 2 fingers on the othr hand, then practice with worse fingers (i.e. little and second little?) i used to do this and pull ups like this.
also, dont try to lock in a certain place, your body will naturally find a position that it is comfortable with.
other than what other people have said and watch out for tendonitis