Meaning of Walt Disney's term "plussing"

I've read that Walt Disney's greatest talent was his ability to get people to do better work than they imagined they were capable of, but it wasn't until I read Ward Jenkin's entry about "plussing," that I knew Disney had a coined a word for it.

Walt Disney coined the term plussing as a way of making an idea even better. By telling his workers to plus it, even when they think they nailed it, gave Disney that extra edge when it came to quality animation back in the day. Pixar is a staunch believer in plussing their work. And it shows.