In 7NT (an overbid by design) I have twelve top tricks and extended menaces in spades and hearts. They lead a club and I peel off clubs putting pressure on both defenders. There is a dimaond Jack menace against the defender with the Diamond queen who will come under pressure in three suits, in principle. Each defender in turn tries to keep three cards in spades and hearts but is overburdened. I envision the six card ending to be something like the following. Now Ace and King of diamonds turns the screw on the partner of the defender holding on to the diamonds who must weaken one or the other of the extended meance suits.
If he weakens the same extended suit his partner has weakened (if he throws a spade, )declarer gets long spade for his thirteenth -( the situation in heartss and spades is symmetrical, it does not matter that for my example West has chosen to weaken spades somewhat unnaturally from his QJ)
So let us say he throws a heart in the diagrammed position on the second diamond. Now the sole guard of hearts is transferred back to the person with the diamond guard. So simply cash Spade Ace King and West is simple squeezed now.
This is a typical hexagonal squeeze. I learnt this lesson of identifying hexagonal squeezes from a player on the opponents' team, Bill Cole, in a Yale Harvard varsity match of the nineties.
The grand slam is a poor proposition, results notwithstanding, because an attack in communication in either extended menace suit will set the hand. A diamond or club lead allows the make.
In practice, the diagrammed position (the most thematic) was not arrived at, as West let go of diamond guard.