It is one of the forms of the art known as "Japanese Sharking". "Topshark" refers specifically to form of it where someone(the shark) pulls down an unsuspecting girls top, exposing her breasts, while an accomplice videotapes the event.
I saw that video on the internet the other day where that girls was totally "topsharked" and her boobs where completely exposed to the public!
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.