Well if it was that surely you'd feel something. The way I do it is to go slow, relax and stroke slowly, and not to tense the pc muscle. Then when I feel my orgasm starting I let go of my dick and squeeze my pc muscle hard for as long as it takes. I usually find that if I'm lay on my bed I end up arching my back hard against my bed, I've found it to be a powerful thing for me. If that is what happens to you, I'm sure you'd know it because you'd really feel it.
Often during this I will get a little bit of cum, but not a lot, so then usually I wait a minute and carry on stroking, using the bit of cum sometimes as lube. I don't usually use lube because I'm uncut so find it better to just stroke my cock and let the foreskin slide up and down, but when I do this multiple orgasm thing I sometimes like to pull back my foreskin and stroke over the head more, only if well lubed though.
I don't do this too often really because it can get quite tiring, if I do then I usually do it before bed. I can usually get about 3 orgasms without ejaculating, and then the final time I tend to do it and when I'm clenching my pc muscle I just grab my dick and squeeze it hard, and that's usually enough to make me ejaculate, whilst still getting the feeling of the orgasm, as it tends to feel better than a 'regular' orgasm.
It's been a while since I tried it actually, might have a go later tonight if I feel like it