The Principle of Restricted Choice is that a player is more likely to have played a particular card because it is a forced, ie restricted, play than because it is a chosen play from equals.
For example:
South, the declarer, cashes the ace and East drops the queen. Assuming that with queen-jack doubleton, East will play the jack half the time and the queen half the time, East is roughly twice as likely to hold the singleton queen as he is to hold the queen-jack doubleton.