With no clear choice of opening lead against no-trump, it pays to go passive when declarer has shown a strong hand and the opponents appear to be balanced. West therefore opts for a diamond lead against three no-trump, specifically the eight, top of nothing. This might be best even from only a three-card diamond suit.
Declarer has eight tricks on this lead and must establish dummy’s clubs for more. He will need an outside entry to dummy, and only the diamond suit will provide.
If East kindly covers dummy’s diamond nine with the jack or the ace at trick one, declarer will make his contract with ease, by playing three rounds of clubs from the top and then forcing an entry in diamonds.
East can place the diamond king on his left after the lead of the eight and therefore must play his three at trick one, preserving the ace and jack over dummy’s queen and 10. Now declarer can never reach the table. With so many points on display for East, it is hard to construct a hand where it is necessary to take the diamond ace and switch to a major — the name of the game here is to prevent declarer from making his tricks rather than actively establishing winners for the defense.
Declarer may try to counter this defense by overtaking dummy’s diamond nine with the king and then leading back toward dummy in diamonds later, after clearing clubs. However, that only costs him the extra undertrick.
The contract can be beaten on a spade lead, but East must be on his guard. If declarer offers up the diamond king on the first round of the suit, it is essential that East duck.