You hold the Ace and Queen of Spades. Your partner, who has already made his bid, leads the Eight of trump, and your right hand opponent plays the Nine in normal cadence. If the left hand opponent holds the King, you still lose it. However, if your right hand opponent has the monarch, and ducks, your Queen will win this trick. Then you will score your Ace on the next round of trump.
Finesses come in all sizes and shapes.
It is the first round of this suit. If your partner or right hand opponent leads this suit, and the Ace does not appear, you must play the King. If the Ace-Queen are behind your King, he is trapped. If you play low, any experienced opponent will finesse if he has the opportunity. The key to success is your right hand opponent, and if he holds the Ace or ducks, you are golden.