Tyler has passed along the YouTube below, which depicts the Delpi C-3 CubeSat transponder in action. A transponder, as described by him, basically repeats just about any kind of signal it receives, which means it can act like a communications device. For example, someone in Kentucky might bounce a signal off a transponder on CubeSat in space to talk to someone in Canada. It works because the satellite is so high both places can see it and use it to communicate.
The technique, incidentally, was used on Balloon-1. As it rose through the atmosphere, we could hear radio operators from several state using the onboard transponder to communicate with each other.