Colin Randall, the paper's executive editor, has just left The National building for good. He is going back to France, effectively retiring, and as he made his final rounds some of the other former Telegraph editors starting slowly pounding their desks with one hand. Soon everyone had joined in, sort of like the lone person clapping in the cliche moving endings, only better.
Colin is a lovely man, even if he does devote entire columns to punctuation and say things like "I thought it was rather inelegant, the way you included his age in the lede" (he was the paper's style guru, after all). And it was quite stirring as he made his way around the room in the growing din, finally walking out in a chorus of whoops and hollers.
Apparently this is a British thing, when you retire. And since everyone is still supposed to be working, they hit the desk with just one hand.