But in cases where the bribe is to expedite an official act—they give the example of getting a driver license issued—why wouldn’t the official just give the smile then shuffle your application to the bottom of the pile?
The official would be wary of appearing to take retaliatory action, since by being given the note, the person would have already signalled that they are the type to actively oppose bribery rather than just refusing to pay and otherwise turning a blind eye.