This coup is also referred to as the disappearing
trump trick. In French the coup is known as le coup du diable. It occurs during a certain configuration of the cards when a seemingly certain trump winner disappears, or becomes no longer a certain trump winner. This situation can be examined with the following illustrations. It is important to keep in mind the configuration of the cards.
Note: In some parts of the global bridge community this declarer action is also known under the designation, perhaps colloquially, as The Smother. Source is an article published by Mr. Carl Edward Dickel of Glasgow, Scotland, who for forty years authored a weekly bridge column in the Glasgow Herald. This article has also only been preserved and archived on this site in .pdf file format for future reference.
See also the French description provided by Mr. Roger Trezel in his publication L'Officiel de Bridge, published in the year 1979. An excerpt is included in the original French in .pdf file format.
Example 1
The declarer is South. The contract is 6 Hearts. After surveying the configuration of the cards, South realizes that one loser is the Ace of Spades, and a second loser is a possible trump trick.
The declarer, South, must make this assumption that the trump split will be 3-2 and that the two missing honor cards in Hearts are held either by one defender or are split between the two defenders. The declarer must make this assumption, since this split is the only distribution, by which the declarer has an opportunity to make the slam. If the two outstanding trump honors are held by the same defender in a 4-1 or even a 5-0 split, then the declarer has absolutely no chance of fulfilling the contract.
| Trick 1: |
West leads the Ace of Spades, which wins. |
| Trick 2: |
West leads the King of Spades, which is trumped by South. |
| Tricks 3-4-5: |
South then plays three rounds of Clubs, ending with the King of Clubs in dummy. |
This play results in the 
4 becoming a winning card, but only after the outstanding trumps are depleted in the hands of the opponents. This card, however, presents and becomes a problem. If played before the outstanding trumps are deleted, then this card will be trumped and the contract defeated. The presumed outstanding trump trick, held by West in this example, prevents the declarer from cashing this card. Therefore this card is ruled out as being a source of a winning trick.
| Trick 6: |
South plays the  5 from the dummy, ruffing in hand. |
| Trick 7: |
South then plays the  4 to the Ace in the dummy. |
| Trick 8: |
Then South plays the 6 back to the King in hand. |
| Trick 9: |
South then plays the 5 and ruffs in dummy. |
| Trick 10: |
South plays the last Spade, Jack, from the dummy and ruffs in hand. |
End Position: The end position for play of the eleventh trick is as follows.
South leads the 10 from hand, and as the configuration of the cards indicate, both West and East can not prevent South from taking the remainder of the tricks. If West trumps low, which he has to do, then South overtrumps with the 10 of Hearts, and collects the last two tricks with the last two high trump cards. If, however, West trumps with the Queen of Hearts, then South overtrumps with the King of Hearts, and finesses East for the Jack of Hearts. The seemingly losing trump trick has vanished, disappeared, and did not materialize.
Example 2
The declarer is South. The contract is 7 Spades. At first sight, the contract seems doomed. But the declarer must give himself that extra chance and the possibility that the trump split is 3-2, with the Queen of Spades in the doubleton. West leads the 10 of Hearts. South wins with the Ace of Hearts, ruffs a Heart, leads a Club to the Ace of Clubs and again ruffs a Heart. South then wins three Diamond tricks ending in dummy.
This means that South overtakes the Jack of Hearts with the Ace of Hearts. South then ruffs a Heart. It would not prove beneficial if East, at this trick, to trump since South then would simply play the Ace of Spades, dropping the Queen of Spades, and finesse West for the Jack of Spades. South then plays the King and Queen of Clubs, discarding a Heart and the winning Ten of Diamonds. After ten tricks, the layout is as follows:
South then plays the 9 intending to trump small. If West plays a small trump, then South plays the 7 from the dummy. If West plays the Jack, then South over-trumps with the King and finesses East for the Queen. And the possible trump trick of the defenders has vanished and disappeared. The disappearing trump trick.
If you wish to include this feature, or any other feature, of the game of bridge in your partnership agreement, then please make certain that the concept is understood by both partners. Be aware whether or not the feature is alertable or not and whether an announcement should or must be made. Check with the governing body and/or the bridge district and/or the bridge unit prior to the game to establish the guidelines applied. Please include the particular feature on your convention card in order that your opponents are also aware of this feature during the bidding process, since this information must be made known to them according to the Laws of Duplicate Contract Bridge. We do not always include the procedure regarding Alerts and/or Announcements, since these regulations are changed and revised during time by the governing body. It is our intention only to present the information as concisely and as accurately as possible.