To put it VERY simply, you flex your Pubococcygeus (PC) muscle by doing kegel exercises, focusing on the growing pleasurable feelings in your loins, until you cum. It takes a bit of practice, and being in the right mindset, but it's a very intense way to cum.
I was actually able to get off hands free the other night, but I didn't have my phone with me, and I was worried if I got up to get it I would "lose the feeling". I realized that night that due to the build up being so slow, and the pleasurable zone being relatively broad, I could easily get myself close then relax to let it dissipate. I don't know if I would consider it multiple orgasms, but it was blowing my mind. I did that three times before losing my patience and letting myself cum. Kind of wish I hadn't, cause it was feeling reaaaally good, and I also made a huge mess in my cage.
These are the only two Mindgasm videos I needed in order to figure this out. It took me about three days to finally get it, but your miles may vary. I'm also told the other lessons (that are behind a paywall) are very good.
Male Voice Lesson 1
Male Voice Lesson 2
If anyone has any more of these kind of videos/recordings, specifically with a male voice giving flexing instructions, I would love to hear them!