Andrew
The "mystery" of back tension was what I finally figured out that forever added an inch to my draw length and gave me the kind of accuracy I had been looking for in my shooting for years.
The best illustration I've seen to explain which muscles we're talking about is from the "Master's" DVD I mentioned before.
Take an arrow shaft, and hold it in front of you at shoulder's height. Grip it with both hands, thumbs touching in the middle of the arrow, and palms pointing down at the floor. Now, pretend like you are trying to pull the arrow in half! The muscles that you feel working in your back are the ones I want to have involved in drawing a bow. Does that makes any sense?
For years, I tried to draw so my middle finger touched the corner of the mouth. That is good for a lot of folks, but for me, it did NOT give me good back tension, and I sprayed arrows all over the place. We are all built differently.
One sentence in something I read changed the whole game of archery for me. "Instead of trying to anchor at a specific spot (like corner of mouth), concentrate on coming to full draw with the muscles of your back. Then, where ever your fingers happen to touch your face is your reference point."
For me, that meant my back muscles had proper tension, my new reference spot (rather than "anchor") was index finger touching my cheekbone, and I drew 29" rather than 28!
The deer wish I had never figured that out!!
Hope that picture helps a little bit...