There are all kinds of inferences a declarer can draw from the bidding or early play of a hand.
For example, consider this deal where South got to four spades on the bidding shown.
West led the king of clubs, everyone following low, and shifted to the queen of hearts. On this trick, declarer made the highly unusual move of playing low from both hands!
As a result, South made the contract. West continued with a heart to the ace, and declarer drew two rounds of trumps, ending in dummy. He then discarded a diamond on the king of hearts and played the ace of diamonds followed by the queen.
When East covered with the king, declarer ruffed, returned to dummy with a trump, discarded a club on the jack of diamonds and in that way scored his 10th trick.the only tricks he lost were a heart and two clubs.
Certainly South’s method of play seems odd, since it appears more natural to win the heart shift at trick two with the ace, draw trumps and take a diamond finesse.
Had declarer adopted this course of play, though, he would have lost a diamond and three clubs and gone down one.
Declarer’s method of play was very well-founded. He assumed from the first two plays that West had the K-A of clubs and Q-J of hearts and was therefore not likely to have the king of diamonds also, given his original pass as dealer.
So, rather than pin his hopes on West’s holding a card he couldn’t have, declarer played East for that card and scored a well-deserved victory.