Instead of a demon who plans when to open and close the door t perfect timings, imagine the demon's retarded cousin who doesn't understand what his smarter brother is trying to do so he just installs a one-way door. This door will only open one way if, and only if, the pressure on one side is enough to open it. While open, the things doing the pushing escape through the hatch into the other room. Since they're done pushing, the door closes and doesn't let them back out.