This deal was played in a BMS (Bridge Made Simple) duplicate game after one of my webinars. This was the randomly-dealt 52-card layout:
South opened 1, and with only N-S vulnerable, West stuck in a 1 overcall (about as minimum as you can get). North bid 2 and South's 2NT was raised to 3NT. I have heard students tell me they didn't lead their suit because the opponents have it stopped. My answers is, "try to knock out their stoppers." Most West players led a spade (some hadn't overcalled and the auction was 1-1-1N-3N). What should happen when East plays the K, winning?
East should continue spades and South ducks again. West must now be on the ball and overtake. Because the 9 is in dummy, South can afford to play the Q and then the 8 to plug away at South's stopper. When South eventually wins the A, he counts 7 top tricks. If he chooses to play on clubs, West gets in and wins a club and 4 spades for down 1.
Let's say that instead, South leads the 9. If West sleepily plays low, the contract makes (maybe even with an overtrick if East doesn't return a club). The 9 would lose to East's queen. Declarer now has 4 diamonds, 3 hearts and the black aces for his contract.
We were taught to "cover an honor with an honor," but not everyone realizes that in this case the "9" is in effect an honor. It can't hurt West to play the J, and it certainly can help. Declarer can no longer make the contract. East has 2 diamond tricks. When he gets in, he can shift to clubs and declarer is at least a trick short.
If West doesn't overtake the spade, or cover the 9, 3NT makes in comfort.