Otherwise referred to as 'cutting off the ring', it's trapping your opponent via restricting movement...so if they're trying to get out of the corner, move in front of them, and if they move left, you move in the same direction etc. RJJ, Sugar Ray Leonard are worth watching for this.
Frazier Ali 1 - Frazier is one of the best ever at cutting the ring off - he has surprisingly quick feet and moves laterally to cut Ali's escape. Yes this is not Cleveland Williams Ali but the way Joe cuts him down when he does try to move his impressive. Also see ANY Ricky Hatton fight - he wasn't a great boxer but he has GREAT ring cutting ability and crazy fast feet. Watch Mayweather try to move round him and then give up and fight him up close. He makes Pacquiao's back touch the ropes within seconds of their fight too. It was the one thing he was truly superb at.
A lot of it involves moving laterally rather than following your opponent around. He moves right you move to your left, he moves left you move to your right, all the time edging forward as you do so.
Young Foreman was excellent at cutting off guys. Even Ali couldn't run from him. Old Foreman was too slow and sluggish of foot to do it, which is why he struggled with guys who boxed him.
I thought Kenny Norton was very good at it against against Ali at least - but then I suppose you had to be good at that to have any kind of success against Ali
