The final paragraph of "After Twenty Years" represents the resolution of the story. During the rising action, Bob meets a policeman on his beat and explains the arrangement he had with his childhood friend to meet at a certain location twenty years after their last meal together. The policeman leaves the man, and another man arrives and gives Bob the impression, without saying so, that he is his childhood friend, Jimmy Wells. The two move on down the street and pause under the lights of a drugstore. At that point, the climax occurs. Bob realizes that the man he has been walking with isn't Jimmy Wells.
In the following paragraph, the second-to-last one of the story, the falling action occurs. The man who let Bob believe he was Jimmy Wells is actually a policeman, and he reveals that "Silky Bob" has been "under arrest for ten minutes." At this point, readers are surprised and probably confused, especially when the policeman gives Bob a note from Wells.
The final paragraph answers Bob's questions as well as the reader's. Bob, "the man from the West," opens the note. As he reads, his hand begins to tremble. This reveals some sort of emotion other than the anger Bob revealed when he realized the policeman wasn't Jimmy. The note, given verbatim, concludes the story and provides the classic O. Henry twist ending.
It reveals that Jimmy was actually the patrolman who first spoke to Bob. He wasn't simply a patrolman on his beat; he was there to fulfill his agreement with Bob to meet him at the appointed time and place. When Bob struck a match to light his cigar, Jimmy recognized Bob as a man on a wanted poster: "the face of the man wanted in Chicago." Rather than arresting Bob on the spot or revealing his own identity, Wells returned to the police station ("went around") and found a non-uniformed officer, "a plain-clothes man," to make the arrest. Wells evidently didn't have the heart to arrest Bob himself nor to explain the situation in person. This means that Wells values following and enforcing the law more than he values his childhood friendship with a man who pursued a very different path in life than Wells chose for himself.