This is when you are riding public transit and you offer someone your seat, only to standnext to them and fart directly in their face.
The kind young man offered his seat on the bus to an elderly woman. Moments later, the look on her face made it clear that she had fallen victim to the flip flop ass drop.
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.