The pitfall block acts like a normal block when placed. If it receives a redstone signal it will fall to the ground like sand. It will also activate neighboring pitfall blocks to fall down as well. This can cause a cascade effect which allows you to use less redstone.
This block can be useful for building bridges that collapse on command, cages that fall from the sky, and I'm sure you will be able to think of some others.The recipe is shown in the video. Visit the minecraft forum thread for discussion and feedback.