I believe "chopping" refers to putting your top about 1/2 way down and "karate chopping" the big wrinkle in the center of the crease in the back window so the window folds neatly. Sometimes in really hot weather this isn't necessary, but it is a good idea each time you lower the top.
You can also pad your window as you put the top down to keep the plastic from sticking to itself. You can either buy a Porsche window pad for $$ or use a cheap terry cloth bath towel and achieve the same protection. After you "chop" the window, lay a towel folded in half so the towel crease lays against the window crease and continue to put the top down.
