It means to be fierce, sharp, or clean
In "<em>The Stranger</em>", by Albert Camus, Meursault describes shooting the Arab after he's already dead as follows:
"I knew I’d shattered the balance of the day, thespacious calm of this beach on which I had been happy. But I fired four shots more into the inert body, on which they left no visible trace. And each successive shot was another loud, fateful rap on the door of my undoing."
He describes it as <em>knocking loudly on the door of his downfall</em>.
I believe your answer would be Kennedy wanted to give an inspiring speech without being long-winded. Hope this helps!
The milk's carton has leaked
