My apologize if perhaps I am using the wrong terms, what I am referring to is the Holland Diplexer that interfaces the LNB and as I understand it passes the power from the satellite receiver to the actual dish. It has a DPD2 designation on it and differs from the Diplexer/triplexers that attach to the receiver. I hope that clears things up.
Holland DPD2 2 Amp Satellite Diplexer
It's a Diplexer. It passes the signal from the Satellite to the Receiver where it meets a Triplexer, in most cases.
The Triplexer separates the incoming signal for duo tuner receivers. The third leg returns the signal back to the Diplexer, where it sends it to the TV2.
That's how it works in most cases. What it's doing in layman's terms is allowing signal to pass in two different directions at the same time on the same wire, also known as multiplexing.
In more exotic applications, it can be used to send signals from two different sources to the same place.
An example would be connecting a Satellite cable to the SAT port. In/Out the Receiver with an OTA Antenna to the UHF/VHF port.
At the receiver, a Diplexer would be placed before the Triplexer in the same order. In/Out on the first Diplexer to the In/Out on the second Diplexer at the receiver. The Sat Port connected the to Triplexer and the UHF/VHF to an OTA Input. For a single tuner receiver, there would be no Triplexer and the Sat port would connect to the input on the Receiver. Like a 211, for example
The same thing has been known to work with an Internet signal.
Diplexers are not able to work with Hoppers