What purpose is reflected in London’s decision to describe the man smacking his fingers against his leg

London's purpose would be to accentuate exactly how numb the man's fingers had become. He wasn't merely rubbing his hands, he was smacking them in hopes of triggering some sort of feeling.

what does the change from stinging to numbness in the man's hands mean? what kind of tone does london's description of the man's fingers and toes convey? why?