How does Tom get out of being switched when Aunt Polly catches him eating jam in the closet?

Tom manages to evade his aunt's switching on this occasion with a trick. He has been warned many times that if he sneaks into the closet and eats jam without asking, he will be punished for it. He also cannot successfully convince his aunt that he hasn't been eating jam, as he is covered in it, with his hands and mouth betraying the evidence of his crime. As such, Tom is left with only one option, and that is to escape the switching punishment by deception.
Even as his aunt raises her switch in the air, which Tom has been compelled to pass to her, Tom calls out to his aunt to look behind her. His aunt, at this point, fears that there might be some "danger," probably just as a reflex, but as she turns around to look at what that danger might be, Tom "fled on the instant," climbs over the fence, and disappears beyond it, escaping his aunt's punishment—this time.
Polly's reaction suggests that this is a common trick of Tom's and that "old fools is the biggest fools there is." Having made her laugh, Tom knows that his aunt now won't have the heart to punish him even when he does return.