Sometimes referred to as "Boom and indeed tish"
Usually used to follow a "joke", quote or comment perceived by the presenters as humorous. The words are intended to replicate the sound of a drum and cymbals in what is commonly known as a rimshot.
For example, at the end of Jarhead, they all say "this is an awful pace, thank god we won't have to come back here ever again." Boom, and, if you will, tish.