I have been reading for years about strengthening the PC muscle and doing 'cut-offs' for multiple orgasms, but it took me a while to get the timing right for maximum pleasure.
When orgasm approaches, I bear down hard on my PC and ab muscles. This extends the orgasmic feeling down through the PC and frenulum, and produces a large blob of precum. After repeating this 3-4 times, it produces a small drop of semen. I then relax a little and continue stimulating: the 'minor' pre-orgasm continues and merges into a BIG one!
The only drawback here is that I do enjoy erupting dramatically, but when I use this technique, it usually just dribbles out.