The game completely abandons the idea of previous titles that the playfield is being pushed down by some sort of mechanical device and instead attaches groups of bubbles to nodes that move downwards. When a node is no longer connected to any bubbles, it will disappear and when all nodes in a level have vanished the level is complete. As a result, shooting a bubble to the top of the visible playfield without striking any bubbles causes it to bounce and travel back downwards. (wiki)