A block slides across the top of the tower. Tap the screen, hit the drop button, or press space to drop it. Anything hanging over the edge is sliced off and falls away, so the next block is only as wide as the overlap you left. Dashed rails show the tower's edges and light up when you are lined up. Land inside that window and it counts as perfect, you lose nothing, and three perfects in a row start widening the tower back out. Miss the tower completely and the run is over.