A passing contact switch is one where the electrical connection is made as the switch is moved and then broken again before the movement stops.
Picture a switch with a big lever coming outof the top, pivoted at the bottom (like a signal cabin). Wire connects to the lever. If the lever is moved, partway along the path it touches a contact and completes the circuit. (Throws the switch or whatever.) It keeps moving and eventually settles at the other end of its path.
Usually, the model version has 2 contacts -- one for each position of the switch or signal. So the first contact just keeps the switch as it was, while the second changes it. The big lever is a visual indication of the switch position.