Backs and asses are nice, but there's so much good stuff on the front, plus you can see her face as she begs with you or curses you with her eyes. Foxy Slave in Girl on the Ladder from Cruel World gives a lovely example.
Yeah, that's called imagination. The actresses, at least in my videos, go through pretty uncomfortable positions and tortures. I guess not many attractive girls would accept really accepting whipping, burning, electro and other tortures for a couple hundred bucks--if at all. The best ones do a pretty spectacular job of imagining and portraying their responses.