The ability of your opponent
Abraham Lincoln said, “Tact is the ability to describe others as they see themselves.”
One skill of a successful bridge player, especially one who plays a lot of Chicago or cut-in rubber bridge for money, is knowing the strengths and shortcomings of his opponents.
In today’s deal, South is in four spades. West leads his singleton heart. East wins with his ace, cashes the diamond ace, then gives his partner a heart ruff. West exits with a diamond. How should South continue?
When North responded with a transfer bid, East doubled to show heart length and strength. South’s jump to three spades was a superaccept, promising four-card support. But with 4-3-3-3 distribution, it was an overbid. He should have had a doubleton somewhere. South must draw trumps without losing another trick. But how? He needs to know the skill level of East. If he is a beginner, declarer is none the wiser. He has to guess what to do. But if East is an expert, South should draw an important conclusion.
Why did East cash the diamond ace? Why didn’t he give West his heart ruff at trick two, get back in with the diamond ace and give his partner a second ruff to defeat the contract?
The only logical answer is that East knows West has only one trump and that defense will expose the spade position when West does not ruff at trick four.
So, after winning trick four on the board, South should run the spade 10 through East. When West does discard, as expected, declarer draws trumps and claims.