Are we talking about the protruding bit of the prostate here?
There is a spot between the bottom of the ball sac and the anus where you can rub and make a man a little silly... this is due to there being a part of the prostate gland that is close to the skin surface at that point. You can also massage the prostate from the other side with anal sex using vibes and other toys.
Its sort of similar to how the g spot is actually the back end of the clitoris (which is actually larger than the average penis just most of it is unseen) where it comes close to the membranes on the inside of the vagina.
If I am right about this... the P spot, like the G spot can be in slightly different places in different people and can have varying sensitivity...