This is a problem from today's New York Post.
You (South) are in a stretchy major suit contract with only 22 high card points. You have four top tricks outside the trump suit, and have just won a critical finesse in your hand by finessing to your king. This bypassed an ace in East, whose early play showed an A-Q and five cards in another suit, but did not open.
If East had as much as a queen more, he would have opened, so all other honors including the queen of trumps must be with west. With five (non-trump) tricks in hand, you need five trump tricks to make game. (There are no other trick taking possibilities in this problem, so focus only on this one assumption.)
You have AJ862. Dummy has K54. Opponents have QT973 between them, with the Q in West, for reasons discussed above.
There are two ways to capture the queen. One is to play for the drop. In the problem, West has Q7, so South makes his (optimistic) contract.
The other try is a "backward finesse." Lead the J of trumps from the South hand, and capture West's queen with the king in dummy (if West covers). Then lead back a low trump through East.
As the cards lie, the backward finesse fails, because you are leading through East's T9 to your A8 in hand. (It would have worked if West had Q9 or QT and East had T73 or 973.) But suppose the 8 were a higher cards, say 9 or T (and East's correspondingly lower). Would the backward finesse then have offered better chances for five trump tricks than the drop?