It tells of his many strange assistants, leading up to the American version of friendly little worker elves. I think kids would be less naughty in the United States, though, if they still had to worry about assistants like Black Peter, who might take them away in a sack instead of leaving them toys.
A couple other great assistants:
Père Fouettard is found in France and Luxembourg, where he's known as Housécker. He is the evil butcher who was forever condemned to follow St. Nicolas as a punishment for luring the little lost children into his shop. His name doesn't translate well, but means "Mr. Bogeyman," "spanking," or "switches."
And then, of course... The Devil!
In Czech and Slovak traditions, a shaggy furred devil with horns, tail, and a long red tongue comes along with Saint Nicholas. He has a staff to threaten punishment. However, he is chained and an angel is there to protect the children.