Second hand plays?
Consider this suit:
The question is: if declarer leads this suit, can he score a trick?
The answer is: No, he cannot make a trick. But only if the defence do the right thing.
First consider if declarer leads ♣3: West must play low; dummy’s ♣Q will be beaten by ♣A and West holds ♣K to beat ♣J. If West erroneously rises with ♣K on ♣3, then that card has taken nothing and now declarer holds ♣J and ♣Q as equals against ♣A: one of ♣J/♣Q to force out ♣A and promote the other.
Now consider if declarer leads ♣J: West must play ♣K this time; then East holds ♣A to beat dummy’s ♣Q and no trick is made by declarer. If, however, West erroneously plays low on ♣J, then East will win ♣A but declarer can later lead towards ♣Q; West can win ♣K if he likes, but ♣Q will win the third round.
We have reached a very important general conclusion for a defender playing second to a trick: on a low card, the second player should play low, but if an honour is led, he should cover an honour with an honour.