As with most people I discovered this completely by accident and I'm not exactly sure how. I've seen a small number of videos (2 or 3) of guys doing something similar so I know I must not be totally alone in this. Anyway, I'm circumcised, and on the underside of my penis is a very very small raised area about an inch below the head, I guess it's a remnant of my foreskin/frenulum. I take one hand (usually my left) and stretch the skin of my shaft away from the head and then use the middle finger of my other hand (usually my right) lubed with Vaseline and rub up and down to the right of that very slightly raised spot. I do this for maybe 10-15 strokes up and down each time and then relax for 10-15 seconds, then repeat. It usually doesn't take long before I start to feel that feeling, you know the one I mean. I like to edge just a little.
When I was younger I could keep myself on the edge for 15 minutes or longer but now I usually just get myself right to the edge of cumming 3-5 times and then back off, just enough to get the pre-cum really flowing (which I love the taste of BTW and always try to collect as much as I can for tasting, but I've never enjoyed the taste of actual cum). Usually after the third time of getting to the edge I let myself cum. These orgasms with this technique are the most explosive, best-feeling orgasms I have and I'm totally addicted to the feeling.