I had one girlfriend that could milk me like that. I"m pretty sure that is what she was doing. I hate so sound like a stereotyping dumb ass, but do women have muscles surounding the vagina that maybe only some realize they have and exercise and strengthen? The G Spot wasn't know about 45 years ago, to my knowledge. And squiring? Forty years ago I remember this one really hot lover and I were wondering why the sheets were soaked at crotch level but never smelled like piss.
Anyway, what I'm getting at is I had this gf 35 years ago that several times would grab my ass and pull me all the way in and hold me there. Then she would massage/milk my cock with her pussy. It was like she had fingers in her pussy that started to contract the little finger, then the next one, and then the next one, etc… It was subtle but I could feel it. I was astonished and asked how she did that. She just gave me a shy look and said that her mother taught her how to use her pussy.
Years latter I met a guy that shad to me that Philippina women had fingers in their pusys.
I could relate, though I didn't like the stereotyping. (reread the third line here).
I hope this helps more that confuses.