This was a grand slam force, asking South to bid seven diamonds holding two of the top three honors.

If South had bid three spades, he would probably have been doubled.

South therefore bid two diamonds, showing length in the suit and positive values.

But South was not willing to settle for that and bid six spades.

If South had bid diamonds, West would probably have led a club or a heart.

South might well have bid two no-trump but cautiously passed.

He knew that South had bid the slam with two or three quick spade losers.

South has bid four spades, prompted by his partner's negative double.

South would bid two diamonds and continue to game after a jump preference of three spades.

Warned by the opening, South would probably have bid four spades.