This is an easier strategy to use on Doubles. In that variation, you can purposely lose your ball before hitting the last brick (before the screen resets). Then the game will start you out with two balls again, and there's a chance one of the balls will get stuck at the top while you hit the other one below, resulting in double points as long as you can keep both going. The Cavity variation doesn't work the same way, as the game doesn't start you off with multiple balls each turn and, most likely, you've already freed and lost the extra ones by the time you'd like to get one stuck at the top! I won't say it's impossible, but it's very difficult with the combination of fast-moving balls and small paddles!