Halla balla is a Norwegian slang for greeting someone in an old jolly way and it is funny say, because it rhymes. But you should not say it in a job interview for example. Halla is slang for hallo, and means hello, while balla basically means balls.
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.