Michael surprised everyone on April 18, 1971, when he appeared on Diana Ross's television special Diana! as a miniature Frank Sinatra, singing "It Was a Very Good Year." Not only was his impression spot on, from the costuming to his smooth moves, it was the first time he had sung solo in public, leading some fans to start speculating that Michael would one day launch a solo career, as his mentor Diana Ross had done. They turned out to be right about that.
By all accounts, Michael was a natural when it came to mimicking adult performers. But he probably didn't grow up watching and studying Frank Sinatra's moves in the same way he did with James Brown and Jackie Wilson. So someone on the production staff of Diana! had to teach him how to do it. This tutorial was captured in pictures with the contact proof sheets below.
I'm sorry I don't know the name of the man who's showing Michael how to do it. If anyone recognizes him, please let me know. He did a great job of schooling Michael in Sinatra, and the results can be seen in the YouTube clip embedded below.