Progressive rocker gives the board a smooth transition in feel as you step back. i.e. as you slowly move back the board gets looser in proportion to your movements. Some board have a real aggressive kick in the rocker resulting in a major change in the boards performance in a very perticular part of the board, where as other movements slightly forward of the this point do not reflect similar change.
Main difference is Progressive rocker boards is they are generally nicer to surf from the middle part of the board (still turn and carve) than a board that has a straighter rocker through the mid - nose area with an aggressive kick at the tail. Boards with the straighter mid-nose and the tail kick are generally tracking and nose riding anywhere forward of the backward surfing position, and only really loosen up for shortboard moves when your right back.
