I've rewatched the pole!sex scene 10 times now looking for this -- expected it to be like the reunion!sex scene where they loop the same bit a few times for effect -- but I gotta say, I just don't see it here. We see B pull his pants part way down, then the rest of the way down. I never see them back up. *shrug*
But I totally see the backpack thing! I can't believe I never noticed that before! It's dizzying.