Val,
I was just wondering, say you'd read some really nice juicy chick-lit novel (I read LOTS, so I KNOW) and then getting 'led there' --- it is then often UNAVOIDABLE that in some sort of "desperation" you start squeezing your thighs together, and then some more, and... ... ... whilst you read on, eh!
There comes a point, and you are not at all touching yourself, that your PC muscle (pelvic floor muscle) start WANTING to contract (Kegel exercise like). You go on like that and... see what happens.
Maybe not the first few time around, but hey.
So, I'm saying that seems to be far less connected to nerve bundles in the, say, the clit?
It is just that if not in a big O mood, there's at best only one very 'flat' or shallow O, as Taka also mentioned.
That BTW ALSO happens to natal guys too.
Axelle