The riff: Intoned in an old-timer voice by Joel as the innocuous girl in this dating tutorial picks up a hammer to nail up a sign.
The explanation: This is indeed a dark riff. It refers to the old days of slaughterhouses, where it was traditional to use a specially devised hammer to knock cattle or pigs unconscious before slaughter. Workers had to be skilled with the hammer in an attempt to keep things as humane as possible. Today, animals are typically stunned via electric current or pneumatic pistol before they are killed.
Novelty factor: I knew what they were referring to, but reading about these things took me down a more grisly path than I was hoping. I don’t recommend it, if you want to keep enjoying your steaks.
NOTE: I’m not sure there are any appropriate visuals for this riff, so I’m leaving it blank.