... is so named because the score includes a sudden fortissimo chord at the end of the opening theme of the second movement, which up to that moment is played on solo piano. The music then returns to its original quiet dynamic as if nothing has happened. This is the most famous of many 'jokes' included in Haydn's music.
In his old age, and in response to a query from his biographer, Georg August von Griesinger, Haydn denied that he'd written the drum stroke (as it's known in German) to wake up the audience: "I was interested in surprising the public with something new, and in making a brilliant debut, so that my student Pleyel, who was at that time engaged by an orchestra in London and whose concerts had opened a week before mine, should not outdo me."
© Haydn Thompson 2021