I would like to know what is your thought on the vertical plane in which the body moves during a pull up. A local coach at my Gym informs that just when the chin is about to reach the bar there should be an effort to keep the body away(5-10 degrees wrt to legs) from the bar so that the net effort looks similar to climbing a wall. In short if you look from the side at the top of the movement the body should be slightly away from the bar. He says if you are very close to the bar you are pulling mostly with the Biceps.
Sorry, if my English is bad.