A large/fat transexual.
Did you see that trannysaurus screwing that donkey?
When your lady is in the "reverse cowgirl" position, you put clench your hand with the middle knuckle raised (similar to the frogging punch). With this protruding knuckle you then place it on her spine and rub back in forth. The feeling is very intense/ticklish. It will cause her to moan in a weird fashion and her arms will become incapacitated past the forearms for a brief moment.
Some women also seem to do weird things with their mouths during the moment.
man 1: "my girlfriend is super mad at me"
man 2: "why is that"
man 1: "I was trannysaurusin' her for a while and now she has chronic back spasms"