Your method of masturbation is called prone masturbating. I did the same thing, specifically I could hold in my semen (most of the time). Some does go back, but the shaft and hidden shaft, ball area, all stays full. I'd get up, pinch, then shoot a large load into a napkin. Sometimes the load would be so powerful, that even with me prone or pinching, it shoots out anyway and I mess up my hand/pillow, I was grinding.

As far as the variation of you laying on your back. It sounds like the same stimulation of prone, except you just do it off your back. The pressure of the grind is what makes you cum, not stroking or the wet lotion on the hand feeling (which as a kid I hated too).

These days I do not prone masturbate and I have had issues with irritable prostate, sensitive urethra, because of years of abuse/scar tissue from grinding. Nothing major though. I do enjoy stroking normal now. At first sex was weird (the feeling -- but not the motion, as when I Prone, I almost humped my hand/pillow) , oral sex didn't make me orgasm, etc. It took a while to get used to the new feels of sex, oral sex. Now, prone masturbation is uncomfortable and I enjoy stroking and all the things sex has to offer.

For a while though, I could make women happy with certain things I could do really easy. Grind their legs, their hands, their feet, and even small breasted women, I could grind their chest and orgasm. Obviously, this was a small breasted woman's "tittie fuck" and it made them feel good. To make a man cum with their chests. Although it was because I could prone masturbate, either way, it worked out. Those were the days I was in between. I could orgasm with sex but still masturbated in my own prone way.