The coup is a trump reducing play, the virtual discard of a trump by the declarer, because he has too many. This sound rather odd.
The only thing that distinguished a grand coup from its humbler brother, the coup, is that in the reducing play it is winners instead of losers that are ruffed.
Let us illustrate a grand coup, where South had to get rid of three superfluous trumps, in order to make his contract of 6
:
 A Q 7 K 3 K Q 4 A Q 9 6 2 |
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| J 9 8 2 — A 10 8 7 3 2 J 10 5 |
10 4 3 Q 9 8 6 J 9 5 7 4 3 |
|
| K 6 5 A J 10 7 5 4 2 6 K 8 |
Contract: 6 by South
Lead: A
The A followed by the 2. At trick two South ruffs his good diamond. This play would be automatic with a first class player in this situation. Now a heart to the King reveals the position, the declarer has still two trumps too many.
A heart is lead and the 10 finessed. Dummy is re-entered with the Q, and the King of diamonds is lead and ruffed. This is followed by the K, and small to the Q, now the A is ruffed with the seven of trumps, leaving this position:
| A 7 9 6 |
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| J 9 8 — 10 |
10 4 Q 9 |
|
| K 6 A J |
Dummy is entered with the A and a good club is played. If East discards, so does South.