If you place a block across from a open trap door (when the trap door is on it's side) then the trapdoor will close (go flat on the ground)!

To reproduce:
Place a trapdoor over a hole for example, and open it , then place a block across from it, the trapdoor will close.

If you still don't understand then I can attach a screenshot tomorrow, if needed, although you should be able to figure it out.