“Your partners for the Yule Ball, Potter,” she said coldly. “Your dance partners.”
Harry's insides seemed to curl up and shrivel.
“Dance partners?” He felt himself going red. “I don't dance,” he said quickly.
“Oh yes, you do,” said Professor McGonagall irritably. “That's what I'm telling you. Traditionally, the champions and their partners open the ball.”
Harry had a sudden mental image of himself in a top hat and tails, accompanied by a girl in the sort of frilly dress Aunt Petunia always wore to Uncle Vernon's work parties.
“I'm not dancing,” he said. ...
I got the definition for "open the ball" from dictionaries: To open the ball to begin the dancing, to begin operations. But I feel "begin the dancing" doesn't sound quite exactly what it means in this context. What does it suppose to mean here?