Here is another situation where a defender must leap up with an honor to grab the lead in second seat.
There is a good case for North to drive the partnership to three no-trump after South has shown a balanced hand, but he prefers the spade game for its power, perhaps fearing diamond weakness opposite.
West leads the diamond jack, and declarer naturally starts on trumps. He wins with the diamond ace as East starts an echo with the nine. Declarer then calls for the spade queen. The spotlight now turns to East, who knows South has only five spades. There is nothing much to attack in dummy, and South appears to have started with the diamond king and queen (since he did did not play low from dummy at trick one). In that case, a third-round diamond ruff is in the offing if West has the spade king. To preserve that entry, East must hop up with the spade ace at once. South wins the subsequent diamond lead, and West takes the next trump trick perforce, returning a diamond for his partner to ruff. East exits passively and must eventually come to a club trick for one down.
If East plays small on the first spade, West loses the entry for the diamond ruff. Similarly, if declarer starts trumps from hand, West must resist the urge to go in with the king (and he should see this — declarer would have started with the spade queen if he had the ace). The key is for East to preserve an entry to partner’s hand.
This defense would be far harder for East if he had the spade king instead of the ace!