I use an assisted chin-ups machine. One of my goals is to get through a set without the machine.
There are 2 separate places for hand placement. The first, about shoulder width apart pulling up. The second has the hands almost directly above your head, pulling yourself up, palms facing each other. It's almost like doing a row only UP (Sorry if this sounds confusing).
I asked the gym trainer what differences there would be. He said that with shoulder width apart and palms facing away from you, you use more of your back muscles and less of your arms (Biceps). Whereas with the palms facing each other style grip, closer together, you don't hit your lats as much and using more arms than back for the movement than you would otherwise.
Any of this make sense?