To break down the word , head refers to getting neck ( having ones genitals sucked). %20bin refers to the amount of nut you have in the "tank". So the term head%20bin simply is defined as , upon getting neck one nuts beforehand ,therefore lasts longer during oral sex.
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.