It is a very important rule in defence, that the defenders play differently depending on which position is their turn:
When we play a card on partner’s lead, then if he leads a small card we must play high in third place, otherwise the declarer could win the trick with a small card, which should not be able to take a trick:
If South leads the diamond deuce, on dummy’s three North must play the Queen, otherwise the declarer would win with the nine, besides his Ace.
However, when declarer leads, the player in second place must play low when he has an honor in a suit, partly to leave the decision to his partner in fourth position and partly not to help declarer.
To visualise this, have a look at a declarer problem:
Declarer needs one trick in this suit. He has a totally equal chance (50%) to play a small card to the Jack, finessing against the Queen or to play a small to the King, finessing against the Ace.
Therefore declarer leads a small card from the table and if West plays small declarer has an even chance if the Ace and the Queen are in different hands! (If both cards are with West then declarer always succeeds, if they are with East, he never does.)
2nd place low:
If now West makes the mistake of playing an honor, declarer could no longer go wrong. Upon the Ace he plays the Jack and his King is high, or he covers the Queen with his King and he is certain of a trick.
That means, that West has to play slow from 1 honor in 2nd position, also a key point!