Diplexors sound fancy, but are pretty simple
Imagine that you have an outlet in your living room, and in your master bedroom, with 1 end of the cable hooked up to the wall plate, and the other end down in the basement, at the hydro panel
Usually, the satellite installer will utilize your existing cable, running a line from the dish to the hydro panel, and then connecting to your existing cable up to the living room
If this was the case, you could use a diplexor
It looks just like a 2 way splitter
It allows for satellite signal to travel one direction, and the output from the box in the other direction
You would have 1 diplexor behind your 5900
It would have one cable going into the 5900 "sat in"
And another cable "out to tv"
The remaining port would be hooked up to the wallplate behind the 5900
Down in the basement, you would have another diplexor
1 leg would be hooked up to the line from the living room
1 leg would be going out to the satellite dish
1 leg would be going to the master bedroom
This situation is nice when you don't want to have to run additional cable, and for a few bucks ($10-$20) for the diplexors, you can have Pvr in another room as well
I hope I have made it simpler to understand

Good luck with your install