The avoidance principle
In many hands, declarer cannot afford to have a particular defender gain the lead and therefore does everything possible not to lose a trick to him.
Take this case where South was in three notrump, got a heart lead and correctly played the queen from dummy. The purpose of the queen play was twofold: If West had the ace, the queen would win, and the K-8 would constitute a stopper if West later gained the lead; if East had the ace, he would win the trick, but then South could duck the heart return, holding up the king until the third round. This would serve to neutralize West’s remaining hearts if he started with five of them.
When the queen held, South cashed the A-K-Q of spades, hoping the opposing spades were divided 3-3 or that West had four of them. In either case, South would then be sure of the contract. But East showed up with four spades, and declarer went down one after East won the fourth spade and returned the heart jack through South’s king.
Actually, the problem of avoiding the dangerous opponent, East, could have been solved in an entirely different way. After winning the first heart with the queen, all South had to do was to lead a low spade from dummy and finesse the eight.
This would have lost to the ten, but West would then have been stymied. Nothing he could do would stop declarer from scoring nine tricks consisting of four spades, a heart, a diamond and three clubs.
Finessing the spade eight after East played low was sure to bring the contract home if the spades were divided 3-3 or 4-2, and therefore offered a much greater chance of succeeding than simply cashing the A-K-Q in hopes of a 3-3 split.