The finesse
A finesse is an attempt to win a trick with a card that isn’t the highest out, by playing it when the danger card is likely to be trapped.
When it is used
The missing honour you hope to capture is equally likely to be in either opponent’s hand, so a simple finesse works about half the time, and half a chance is much better than no chance at all. It is a key way to make extra tricks.
Always lead from the weak hand toward the strong hand. Lead the queen from dummy first and West sees it before playing: with the king, West just covers, and you win nothing extra.
An example
Dummy has ♠ A Q 6 and you have ♠ 7 3 2. The king of spades is missing.
Lead a small spade from your hand toward dummy. If West plays low, play the queen: it wins when West has the king, and you take two tricks. If East has the king, the queen loses to it and the ace still wins one.
Where we teach it
Taught in the Beginner course, in the Declarer Play Techniques topic, in both systems, ACOL and 5‑card majors. Improver returns to it in Finesses and the Cross-ruff.
Learn it properly
Bridge Unlocked teaches the game step by step, with real hands to play. Bridge Basics is free.