How did Tom manage to escape Aunt Polly's hold in The Adventures of Tom Sawyer?

In the opening chapter, Aunt Polly is trying to get hold of Tom. But first, she has to get her spectacles on, and then she mistakenly pokes under the bed before finding Tom hiding in a closet. She discovers he has been eating jam, because it is all around his mouth.
She grabs a switch and tells him she is going beat him, because she has warned him over and over to stay out of the jam. Tom is desperate to get away from the switch, so he uses a stratagem. He tells his aunt to look behind her, as if something is there:

“My! Look behind you, aunt!”

Aunt Polly falls for the trick and turns around, pulling her skirts out of danger (she perhaps thinks Tom has seen a mouse). Tom immediately runs away, climbs over a high wooden fence, and disappears.