In the image below, you can see the door slides, the door, and the two swinging latch blocks. The latch blocks are only laying in place right now. They will be hinged at their outside-top corners to allow them to swing in and out of the latch notch in the door slides.
The string will then be attached to a hockey-puck shaped piece of wood, which will left both latches, causing them to swing out of the way. The latch blocks will then be stopped by another block attached to the door (not shown), which will then cause the string to lift the door.
The point of this is that the door cannot be opened without lifting it by the string. Raccoons beware!