I don't get many chances to correct Bill, so I'll take em when I can get em.
What he's referring to is really called a switch (multi-switch to be exact). A diplexer is another widget that looks similar, but it's used to combine (and split) different frequency signals onto the same cable. Usually it's to combine an antenna signal and a sat signal on the same cable, to go to a TV/receiver where there aren't enough cables to do it individually. Really useful when you have a big multi-switch (8 or more ports) which also has a built in diplexer.
Here's the gory details on the LNBs and switches. This applies to older DirecTV and Dish hardware. There are additional wrinkles with the newer stuff, and it's different between the vendors.
The satellites transmit (up to) 32 different beams from each satellite location, on 16 frequencies. They can double up on frequencies because half of the beams are clockwise (CW) polarization, and half are counter-clockwise (CCW) polarization. Each LNB can only receive one polarization at a time, and the receiver tells it which to do with a signal on the coax going to the LNB. Most of the LNBs you see are "Dual LNB"s, which means there's actually two in there, which is why there's two coax outputs.
Each coax can go to a different tuner (two receivers, or a dual tuner PVR type), and everything is fine. Each tuner can command it's private LNB to whatever polarization it wants depending on the channel it's trying to tune in.
Now, suppose you want to add tuners. You can't split the coax, because the tuners will fight with each other for control of the LNB. Enter the multi-switch. There's two inputs on the switch, one for each LNB. The switch will lock one input to CW polarization, and the other to CCW polarization. There's multiple outputs, usually 4 or 8. Each of those outputs will be connected to a tuner. When the tuner sends the signal up the coax to the LNB, it only gets as far as the switch. The switch connects that tuner to the appropriate LNB, depending on which polarization it's looking for. Internally, the switch is also a splitter, in the (common) case where more than one tuner wants the same signal.
The newer DishPro stuff does things a little differently, by stacking the signals onto the cable, so you can get both polarizations. But that requires special LNBs with the built-in stacker, or an external stacker (and de-stacker) on each end. Then, there's special 4 input switches for the 101/101/119 three LNB head that DirecTV used to use.
The new DirecTV heads that do both Ku and Ka do things a little differently again, and I haven't researched the details. I know they have 4 outputs, and there are 4 input multi-switches for them, so I think they do things very similarly.
Saydiver just asked me if I was writing a book. I guess so.