Use this term after describing something you used to do, or before you describe something that happened to you. The phrase can be said just by itself if in the right context, to mean "Oh yeah we used to do that all the time"
"Hey remember when we used to always play poker on Wednesdays?" "Dude, Back in Nam"
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.