A transponder is just a wideband transmitter on the satellite. If the channel they are sending through the satellite is narrower (less bandwidth) than the satellite transponder, they can multiplex as many as they want together until its full and send them at the same time. Sort of like 10 computers in your house can use one internet connection. The channels are assigned a Program ID, so the receiver at the other end knows which one of the several channels on the transponder you are wanting, and uses that much of the signal. SD is so narrow, you can usually fit most all the locals on one transponder.