When you stand in a doorway and close the door, you go into the swimming position. Once you move out from the door's block, you stand again. This works with all doors, including trapdoors. If you place slabs above the block you're in (so it's one block tall), you can move around in the 1 block space. You can make the tunnel longer and move around. You can even go through a space that has been filled with 4 snow pieces. However, it does not let you go over a slab.
 
Replication: stand in the path of where a door will close and occupy the block you're in. Close the door. You should go all the way down to the ground and into the swimming position.
 
Also, if you jump, close a door, then jump again, you sort of bounce/spring to the top.
 
Video: https://www.youtube.com/watch?v=F43AqnnYNzQ