Expression used when somethingboss happens, and no other word can explain the feeling. Also appropriate in really ANY situation.
You see someone wipe out - you say bamsters.
You get an A on that test - you say bamsters.
no one is listening to you - you yell BAMSTERS!
awkward silence? bamsters.
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.