Trapdoors

When trap doors are up (open) you press a button and the trapdoor will go to the down (closed) position. They will not come back up.

So,

[] = Button
[-] = Open trap door
[=] = Closed trap door

First:

[]
[-]

when you press the button the trap door goes into:

[]
[=]

and won't pop back up into it's open position