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?
I owm a Adcom gfp-750 with processor loop, basically you use 1 set of cables out of the Adcom into your processor and another set out of the processor into the preamp. When you are watching movies you turn on both processor and pramp, engage the proc loop switch so you can hear your main speakers during movie use. You do not use the proc loop for 2-channel, the preamp operates normally,and the processor does not have to be turned on. An advantage of the Adcom as well is a passive/active switch for 2-channel (active boosts the gain by a few db's) Hope this helps!
I am in a dilemma of trying to do this also.

I have a FT Audio Passive preamp and a Lexicon CP-3Pro which I plan to use.Amps are a Modified B&K ST-202 and a B&K AV2500 Series II for the CC and the rears.

The logic is to allow you to keep your 2ch. seperate from your HT playback.It is my aim to do exactly that.

The question I have is seeing that I have 2 outputs on the FT Audio do I use one for the ST-202 and the other with a "Y" connector for the Sub and the processor?

If anyone can shed light on this please do!

I own both the Adcom GFP750 and a Muse Model 3 Signature, both of which have processor loops. They work as Zackmun said, in that the processor loop is totally separate from the 2 channel electronics in the preamp, except when the processor loop is engaged the volume is controlled by the processor and it passes through the preamp at a "unity gain," which then drives your two-channel power amp and speakers as the two front speakers in your HT set-up. When the processor loop is off, no signal passes through the preamp from the processor. Both of those preamps work very well in that regard, and make it super easy to incorporate your two-channel music speakers into your HT system, without adversely affecting your two-channel sound. My processor is the Lexicon CP3+ which is perfect for incorporating everything since it has 7 channel outputs plus a subwoofer output, and allows different gain settings for each output. However, since it is an older unit, it does not incorporate the latest processing software. Trust me, these preamp processor loops work well and are not hard to set up if you follow the manual. All that is required for the Muse is a single patch cord from the processor's main L&R output. Good luck.

Why do you need a cable sending signal from the Preamp to the Processor? I'm not sure why this would be necessary. Also, I've never read that on the Adcom preamps that the processor loop provided a Unity Gain pass thru. Does this mean the volume control does not effect the Main L and R outputs to the amp when the processor loop is turned on?