The term "flanging" comes from one of the early methods of producing the effect. The finished music track is recorded simultaneously to two matching tape machines, then replayed with both decks in sync. The output from the two recorders is mixed to a third recorder. The engineer slows down one playback recorder by lightly pressing a finger on the flange (rim) of the supply reel. The drainpipe or subtle swoosh effect sweeps in one direction, and the playback of that recorder remains slightly behind the other when the finger is removed. By pressing a finger on the flange of the other deck, the effect sweeps back in the other direction as the decks progress towards being in sync.