Discuss one flashback from "The Last Leaf" by O. Henry.

This is an interesting question dealing with O. Henry's technique, and one that has not been asked before. O. Henry handles his flashbacks adroitly and unobtrusively. One flashback explains the relationship between the two main characters, Susie and Johnsy. Another flashback seems necessary to explain when and how Johnsy caught pneumonia. The next is used to suggest Johnsy's fatalistic state of mind, which is essentially what the whole story is about. The next introduces Old Behrman, who will be very important to the story, although the reader does not realize this at the time. The final flashback will clarify the mystery of why that last leaf so steadfastly refuses to be dislodged by the storm, as if it is clinging to the vine in order to set an example of courage for the sick, languid girl who keeps watching it.

There is a flashback to May in the following exposition:

They had met at the table d'hote of an Eighth street “Delmonico's,” and found their tastes in art, chicory salad and bishop sleeves so congenial that the joint studio resulted.

That was in May. 

The narrative continues as a flashback, telling how Johnsy caught pneumonia in November.Then it resumes in the present when the text reads:

But Johnsy he smote; and she lay, scarcely moving, on her painted iron bedstead, looking through the small Dutch window-panes at the blank side of the next brick house.

There is a flashback in the form of dialogue when Johnsy tells Sue:

“They're falling faster now. Three days ago there were almost a hundred. It made my head ache to count them." 

There is another flashback when the narrator introduces Old Behrman.

For several years he had painted nothing except now and then a daub in the line of commerce or advertising.

And finally there is a flashback in the last paragraph of the story when Susie, having waited until Johnsy is stronger, tells her about what happened to Old Behrman. The entire paragraph is not a flashback but mainly the following sentences.

The janitor found him on the morning of the first day in his room downstairs helpless with pain. His shoes and clothing were wet through and icy cold.

Evidently this is a flashback to an incident that occurred two days ago. Behrman was an old man and did not last long. He had caught pneumonia when he went outside in the bitter cold to paint his masterpiece, a perfect ivy leaf that could not be blown away by the strongest wind.