The Grand Slam Force is a convention that uses an artificial 5NT bid to ask partner about the quality of his trumps, for the purpose of probing for a grand slam. After a trump fit has been found (or implied; 1
:5NT would indicate a heart fit), any 5NT bid that does not conflict with other conventional meanings a pair has agreed on (i.e. a 5NT Blackwood rebid) asks partner to bid a small slam with less than two of the top three honor cards in his suit, and to bid a grand slam with two top honors.
The grand slam force is usually used when one player has one top trump honor and is certain all the other suits are completely stopped, but needs to find the other two top trump honors in partner's hand in order to count 13 tricks.