An unfortunate and potentially traumatic experience which occurs when a heterosexual male is masturbating to a porno flick, and notices that the porno flick is about to enter the money shot scene. The masturbator furiously races to match the timing of his ejaculation with that of the porn star's facial spray onto the cum-craving whore. However, just when the masturbator is about to climax, the scene inexplicably cuts from the gaping whore to a close-up of the porn star's sweaty face as he is jizzing. As a result, the masturbator ends up draining his willy to the porn star rather than the cum slut, leading to a deeply unpleasurable orgasm, severe long-term emotional stress, and a dark era in the masturbator's existence as he questions the meaning and purpose of life. The porn star's face is generally accented by a visible vein popping from his beefy forehead, hence the term Vein Drain.
Dude, I was jerking off to "Butt Fuck Sluts Go Nuts Pt. 9" last night, but just as I was about to erupt in sludge, the camera cut to the roid-head, leading me to vein drain. I have decided to leave all my belongings to you in my will.