This coup is a clever ploy aimed at throwing an opponent in, but only when it is safe to do so. Sometimes this may involve eliminating all his safe exit cards first in order to minimise the dangers that exist. This manoeuvre will either set up a free finesse and/or provide declarer the lead he so desperately craves. In the hand below, many pairs would never be in 6H, let alone make the contract, given declarer's LHO opponent is sitting there with QJ10...AJx...J8...K109.
In dummy there is K.....K10xx....AKQ10x,,,,,AJx
Declarer's hand is Axx.....Q9876.....xx....QxxWhen West leads the queen of spades, taken in dummy, it is imperative that declarer plays out the AK of diamonds straightaway.............before leading a low heart to his queen, and West's Ace. Because what can West do now ? Absolutely nothing. A spade puts declarer in plus a club discard from dummy. A heart gives declarer a free finesse which he wanted to do anyway. And a club gives declarer a free finesse not to mention instant access to hand to finesse the hearts. After any of these choices, declarer has a simple task of making all 12 tricks.
So why is this coup called the Dentist's Coup ? Well, declarer has set about extracting an opponent's safe exit cards as if they were his teeth. In other words making him " toothless ".
However, the next hand is an obscure variation on that theme. Here declarer is in 4H, and his LHO has an impressive Q10xx....Kx....10xx....AKQx.
In dummy there is AK....Q108....AKJxxx....Jx
Declarer's hand is xx....AJxxx....Q9.....xxxx
Best defence requires West to kick of with the AKQ of clubs, forcing declarer to ruff the 3rd round in dummy. Applying the Dentist's Coup he must play the Ace of diamonds first before crossing over to the queen. ( East following both times ). Then when a heart is played towards dummy's queen, West will swiftly rise with the King only to fire back another club which declarer has to ruff with dummy's queen. Now when a top diamond is played from dummy, sure enough East can ruff it.....but declarer of course can over-ruff to see the contract home.
If, however, at trick four a foolish declarer came straight across to the diamond queen first, desperate to play a heart up to dummy's queen, West's fourth club would now enable East to lob away his last remaining diamond. So with declarer locked in dummy and unable to get across to hand to take out the remaining hearts, he will succumb to an embarrassing diamond ruff by East. Here the 4th club posed a very serious threat. Therefore by Playing the Ace of diamonds before crossing over to the queen, the Devil's Coup had proved to be a masterful example of forethought and planning. West in effect had been rendered " toothless ", and his forcing defence in clubs to set up a ruff for partner had been well and truly thwarted.