OK, I'll try. I generally put the pressure from the outside just behind the scrotum or in front of the anus. With a well lubricated (emphasis on well lubricated), you can insert the finger a couple inches and press forward.
This should come later in foreplay. If you two are exploring, just play around with tickling and nibbling. I have had men who particularly like tongue baths of their balls and this would be the extreme rear position for "bathing." The other great spot is the base of the penis right where the scrotum begins.
If you put the finger in, move it gently and press toward the front of the body. When ejaculation begins, get in rhythm. Lots more gets squeezed out.
OK, I have a finger and I have some experience. I do not have a prostate (thank goodness, I like it the way it is!) so men can give better answers.
The book, the Joy of Sex (two volumes; now several years old) covers all sorts of interesting techniques including insertion of things into the anus and removing them as ejaculation takes place.
Men?
