I've just read the linked story which reminded me of my first ejaculation.
I had been masturbating to orgasm since I was nine and having strong dry orgasms. I learned the art from an older boy who taught me to grasp my three inch hard penis with my hand reversed so that my thumb was against my body.
As I approached puberty and my penis got larger, I began using the usual fist grip. But it was not in that way that I first ejaculated. That fateful event happened one day while I was leasurely reading and absentmindedly rubbing the head of my hard penis on my leg. It felt good, but not spectacularly so until I felt a new sensation. On one of the strokes, I felt a sensation which seemed to travel upward from my penis to my navel. This surprised me, so I repeated dragging the head of my penis over my leg. The sensation repeated itself.
I went to the bathroom, removed my clothes, sat on the toilet, rubbed the head across my leg while I watched, and the penetrating sensation happened again. I kept doing this and watching my penis swell to the largest hard I ever had till then. The sensation now permeated my entire lower abdomen. I began to stiffen, and suddenly I felt the unmistakable beginning of an orgasm.
I was now beyond the point of no return with my penis throbbing and moving on it's own. As the sensation peaked, the meatus opened and with each spasm a drop of white sperm emerged. I felt an electrified sensation over my whole body which peaked with every rub. I continued rubbing to keep this ecstasy, but had to stop because I couldn't take any more of it. Afterwards, I slumped still sitting on the toilet in a dreamlike reverie until at last I recovered enough to clean up and re-emerge into the live a day world. I had crossed another of life's thresholds.