What is the catch to the dance instructor job that Ruby applies for in the novel, Ten Cents a Dance?

Ruby jumps at the opportunity to become a dance instructor, but she soon learns that the title of "instructor" is a misnomer. Ruby quickly discovers if she treats the men coming to the taxi dance hall well, they not only will dance with her for a nickel a dance, but they will also tip her well and take her out on the town, often paying for restaurant meals and give her spending money.