A grand coup is defined as a play, by which the declarer intentionally shortens his trump holding by ruffing a winner in order to achieve a finessing position over an adverse trump holding in an end position.
The following example of a sextuple grand coup is from the publication Bridge á la une authored by Mr. José Le Dentu in the year 1964, published by Fayar of Paris, France. LC: 66055562, ASIN: B005ESHNOY.
Quoted from the publication.
South plays 6 redoubled |
West leads:  King |
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Declarer needs to reach a three-card ending, in which both he and West have only trumps left. To that end, the declarer ruffs the lead in dummy and underruffs with the 2 in hand.
He ruffs the 
Ace and then goes back to dummy four times in hearts, which allows him to ruff five clubs in all. Now he can exit with the Jack and West is endplayed in trumps. Note: the initial underruff is not, strictly speaking a Grand Coup, but there are still five of them!
End quoted text.
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.