When you beleive that there is deception or foul play afoot, and you want to be the one that calls it.
You stand up, point at the object/person that you believe to have been involved in the deception and yell "Shannanigans!!" as loud as you can as to attract attention.
However there is an element of risk involved. If you are wrong about the "Shannanigans" call and there is nothing sinister about the situation, you will look like a douchebag
everybody had a hearty chuckle at Jerry when he called Shannanigans on Tom, thinking that he has slept with his wife. Only to find out that he was dead wrong.