Kommentar zu Niddah 5:2

שנזדעזעו איבריו – that semen was removed from his body.

If he was eating terumah when he felt that his limbs shook, he should take hold of his member and swallow the terumah. If a man is eating terumah and he feels himself about to ejaculate (his limbs are shaking) he should squeeze his penis so that he doesn't ejaculate quite yet and then swallow the terumah. If he ejaculates while terumah is still in his mouth, the terumah will become impure. [I realize, this is a bit strange and I'm not sure if it's realistic. It would also take a large degree of control to accomplish this, but I guess anything's possible.]

אוחז באמה – and even though that whomever grabs hold of his membrum virile and urinates it is as if he brings a flood to the world, here since semen was removed, it does not return and he warms himself (i.e., masturbates) to immediately remove more at that time.

And it conveys uncleanness however small the quantity, even if it is only of the size of a mustard seed or less. Any amount of genital discharge (zivah, menstrual blood and semen), even an amount smaller than a mustard seed, defiles.

ומטמאין – flux and semen,

At a miniscule amount, to a person that it comes out from. But coming into contact [through touching], that person is not impure until he touches a lentil’s bulk (i.e., usually, the standard size, which is the Egyptian lentil. See Tractate Kelim, Chapter 17, Mishnah 8).