testosterone concentration? are you kidding me? where the fuck are you going with that? a harder dick? the sensation of cum inducing an orgasm? your cock alone is only so much your technique, which isn't much, all you can really do is alter thrusts or mix positions, maybe tilt your body a bit. most of the time it's because women are cock hungry as always, but too lazy and stupid to get the fuck into it and move around.
anal stimulation works because of you know, the loads of nerve endings clustered around the vagina, with the ass a mere two inches away.