The diagnosis of the position of key cards from clues arising from the bidding or play.

For example:

As South, you play in a spade contract after West has opened 1NT (12-14) and East has shown a heart suit.

West leads the king of hearts and continues with a low one. East wins with the ace and continues with the nine. From the bidding you place West with a heart shortage (rather than the king-queen of hearts).

Counting the missing points then enables you to place the rest of the high cards. You place West with the ♠Q, the Q and the ♣A-J. This tells you to ruff with the ace, finesse West for the ♠Q and then finesse in clubs rather than diamonds.