Just took this off another Forum

But in short the prostate is located behind the pubic bone and is most easily accessed by sticking a finger in the rectum and several inches in feeling toward the front, toward the pubic bone. There will be a walnut sized gland that you can feel and mashing it between your finger and the pubic bone is what is called a "prostate massage". Milder stimulation of the prostate and some of the penis before mashing down to milk it will produce in most a discharge of semen without ejaculation or the sense of orgasm.

I didn't do the research but there are some web sites who might have further information:

http://public.diversity.org.uk/deviant/
http://www.leatherquest.com/main.htm
http://www.sexuality.org/index.html
http://www.frugaldomme.com

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Susan