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 Q 4
A Q 9 6 2
|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
Opening 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:
|J 9 8
Dummy is entered with the A and a good club is played. If East discards, so does South.