Diplexors basicly turn a coax wich is normaly a one way path into a pathway allowing for 2 diferent signal types to travel on the same path without "colliding" with each other, it also allows for the diferent signals to travel in oposite directions.
Think of a one way street and theres a tractor trailer traveling down it followed by and preceeded by other like vehicles with only an inch of space between them. Your in a station waggon with the little nuclear family unit trying to get on this road to start your summer vacation to wally world. You would think that you cant get on but you push the diplexor mode button on the dash and your family truckster phases slightly out of the world and you easily slip into the road in the same spot that the truck is in and both vehicles travel safely down the road without harming each other, the same diplexor button will allow you to travel in the wrong direction down this road without anyone getting hurt.