When the window slams shut on Tom Benecke after he has retrieved the yellow sheet, he stares at the "glass between him and the room just before him." In this sentence, there is a meaning that is both literal and figurative. For, Tom envisions himself ahead, through the window, rolling on the rug, grabbing at tufts of it in his joy. Figuratively, Tom looks into the room and contemplates both his future and his past.
realization...that he might have to wait there till Clare came home,...withdrawing her key from the lock, closing the door behind her, and then glancing up to see him crouched on the other side of the window
Tom knows that she would be unable to open this window without help. Anyway, she will not be home for four hours, and he cannot wait so long in such a crouched position. When his attempts to draw attention by dropping coins to the street and lighting pieces of an envelope with a match fail, Tom figuratively sees the "room before him." It will be empty. As the story is suspended while Tom searches his soul, the reader wonders how long he can grip the window with just his fingertips and what action Tom will take when "with all the force of a revelation," Tom knows that all he ever will have had of life will abruptly end: "nothing, then, could ever be changed." Intensely reflective, Tom wishes he had gone to the movies with his wife; he rues the many hours he neglected her as he toiled on his business project in order to attain a promotion:
Contents of the dead man's pockets, he thought with sudden fierce anger, a wasted life.
As his thoughts advance, Tom's revelation finally provides him the impetus to make a plan to get through the window. He is "simply not going to cling" there until he slips and falls; he must make his life worthwhile. Jack Finney's character rejects the prevalent 1950's drive for corporate success and materialism in "Contents of the Dead Man's Pockets," breaking through the glass of narrow-sighted ambition, entering his home. He puts on his coat and leaves to find his loving wife; the yellow sheet of corporate success wafts again out the window, but alone.
The main flash forward (or at least the most vivid one) in this story comes when Tom is out on the ledge and has already retrieved the piece of paper. At that point, he has started to panic. He sees himself losing his balance and flailing, he sees himself falling, hears himself moaning with fear.
This flash forward actually has a very beneficial effect on him. It allows him to "shut his mind" against all of this kind of thought. He is now able to concentrate on the little, step-by-step (literally) motions of making his way back to his own apartment window.
In addition, the flash forward adds to the suspense. We already know that the title of the story implies that he dies, and this flash forward makes us expect even more that this will happen.