No, there isn't a punchline: the answer is a knowledge of the basic blues format and the ability ... a 'turnaround' - the bit at the end of a 12-bar progression when everything is gearing up ...
Blues in simple time with four beats in the bar and mostly follows a 12 bar blues chord structure. 8 and 16 bar blues can also sometimes be found. Blues verses have a three line structure - AAB ...