Most jumps are either 2 seconds (2 sec up, 2 sec down) or 4 seconds. 8 seconds is sometimes little long for my personal liking but it is perfectly acceptable....esp when the song warrants it also.

Popcorn jumps are up/down, up/down, up/down etc. jumps of about 1 second each (up and down!). The problem is 1.) you quickly lose your form when you are doing a move that quickly and 2.) you don't build the strength the move is intended to build because you are not resisting well enough -- you are simply using gravity and a "bouncing" momentum to get yourself up and down.

Hope that helps -- I have seen some instructors do them and I just stick to a 2 sec up and 2 sec down so I am not "too off" from the rest of the class.