You have to have a "High-Flyer", then have to hold the dunk modifier (i dunno on PS2 its L1) So youre holding L1 and press some button i press square. Sometimes he does a facial or he jumps over the player Ala VC. It depends though.
And about the alley-oop i think you have to hold triangle and move the right analog stick or smth i can't really recal because its hard to do in a game.
Sorry this was based on the PS2 control
