Basically, a small plastic switch that operates like tire spikes (minus the tire-destroying aspect of course). One side is curved and the other side is a straight edge. The switch is pushed down when force is applied to the curved side, but not when applied to the vertical side, thus forcing one-way movement. Here's a sketch roughly demonstrating what I mean:
http://i.imgur.com/ZHBavUJ.png
Sorry if my description isn't clear, and any help is appreciated.