What you've heard is referring to prostate stimulation. Some call it the 'male G-spot', but I disagree with that label, as I don't find it THAT pleasurable. Feels good, but its not a G-spot the way I understand a woman's G-spot is.
Anyway, you can reach the prostate by inserting a well lubricated finger in the anus and into the rectum, then press toward the front of the body. It's about 2-3 inches up, or about the full length of an index or middle finger.
With a finger against the prostate or massaging it, the guy's orgasm will be more powerful and longer, and he will likely ejaculate more than normal. Plus, having something in the anus during orgasm makes the orgasm more intense as well.
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Finger in the butt 
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