South dealer; North-South vulnerable.
* Artificial and forcing
Opening lead — 
3
When a player not on lead doubles a slam voluntarily reached by the opponents, the double nearly always is conventional. In effect, it tells partner to make an unusual lead and indicates that the doubler expects to defeat the slam if partner finds the right lead. There is ample Justification for this convention.
Ordinarily, when a pair undertakes a slam, they either make it or go down one, and there is little to be gained by doubling. But if a double by the player not on lead is reserved for cases, where only an unusual lead will stop the slam, the gain is enormous when the double is successful.
Thus, in the present case, if East’s double induces a spade lead that East ruffs, and this is followed by a diamond return and another ruff. East-West score a two-trick set instead of being on the wrong side of a vulnerable slam. East’s double clearly requests a spade lead.
A diamond or club lead would be normal on the bidding, so the double bars either of these leads. West did lead a spade the three — which East ruffed, but East, faced with a difficult choice of returns, shifted to a club.
As a result, South made the slam. Oddly enough, the principal culprit in the misdefense was actually West, not East. West should reason that East might not know what to return after trumping the first trick. He should therefore lead the nine of spades initially rather than the three as a suit-preference signal to East, asking for the higher-ranking of the two remaining side suits to be returned. With this cue to guide him, East would have had a much easier time finding the winning defense.