If a player has the choice between cards, it is sometimes better to play the one that his opponent knows he holds.
For example:
In 1, South, as declarer, plays a small card and successfully finesses the jack. When the ace is played West should drop the queen (the card he is known to hold) presenting South with a choice of plays, finessing East for the ten or playing for the even break.
If West follows with the ten on the second round, declarer will inevitably play for the break.
In 2, this is a side suit in a trump contract. If declarer finesses dummy’s queen and cashes the ace, West should drop the king.
Declarer may then fear an overruff on the third round of the suit. So long as West retains the king, declarer can safely ruff low.
