Processor Loop for 2 channel?

I keep seeing 2-channel preamps (Adcom, McIntosh, etc.)advertising a processor loop for integration with a home theater set-up. My problem is that I don't see how a loop would work. The idea of using a 2-channel preamp would be to have a "better" preamp then the one in a Preamp/processor and route 2 channel music through it instead of the processor but then use the pre/pro when watching movies or other multi-channel music.

I've seen a few preamps advertise a "unity gain" or HT pass-through and this makes sense. I just don't see how a processor loop would work. Can anyone explain this to me?