I don't have the strongest of PC muscles (yet), but I have managed to, accidentally, have an orgasm without ejaculating on 3 or 4 occasions while masturbating. That allowed me to continue masturbating until I eventually ejaculated, so it counted as 2 orgasms in each session.
I can tell you that the sensation is a bit weird at first, since you're not expecting it, but the pleasure you get from it is awesome (plus the joy of reaching orgasm and maintaining a full erection, well that is beyond description).
What happened to me was that I was masturbating the usual way but, for some reason, when I came, all I ejaculated was pre-cum, and no semen at all. And yet, the physical sensation was practically the same as with any other "normal" orgasm. Since no semen came out, my erection remained as it was and I was able to keep going.
My idea of having multi-orgasms by keggeling is that, when you keggel during orgasm, you manage to hold back the semen flow (as you do when you're holding back the urine flow while emptying the bladder), but are still able to feel the same pleasurable sensation (maybe ejaculating only pre-cum). Since no semen comes out, the erection continues and you're able to keep the action rolling.
But this is just my idea. Some more input from anyt multi-orgasmic guys out there would come in handy
