Typically, when declarer has a 2-way finesse for a Queen, each defender has a 50% chance of holding the Queen. The Vacant Spaces Principle is used to recalculate the odds based on how many cards in a defender's hand may hold the Queen. At the beginning of the hand,, each player has 13 Vacant Spaces which may hold the missing Queen. The auction and the play may tell declarer a particular defender has fewer Vacant Spaces which could potentially hold the missing Queen. The hand below illustrates the principle.
Declarer has a 2-way finesse for the ♦Queen. West's weak 2♥ opening bid suggests, West has 6 hearts and East has 3 hearts. When declarer draws trump, declarer learns West has 3 spades and East has 1 spade. Since West has 6 hearts and 3 spades, West has just 4 Vacant Spaces in their hand which could hold the ♦Queen. East has 1 spade and 3 hearts. So East has 9 Vacant Spaces which could hold the ♦Queen. This makes East a 9 to 4 favorite to hold the ♦Queen. The Vacant Spaces Principle suggests declarer finesse against East for the ♦Queen.
See the Vacant Spaces Wikipedia page for more on this topic.
