More Secret of NIMH stuff.
When Mrs. Brisby first enters the rose bush (the home of the rats of NIMH), she pushes aside a branch to get through a hole. This branch is connected to a door by a string and pushing the branch causes the door to open.
The trap is simply a similar set-up where an obvious entrance opens but it is an incorrect, overly long, booby-trapped, or otherwise deleterious path. The actual entrance is hidden by the very act that makes the alternate, incorrect path obvious (such as the door swinging "open" to the wrong path and thereby covering the opening to the right path).