Experienced players are always on the lookout for ways to let their partner know what cards they hold. When someone names trump, the first thing they want to know is where the other trump are. When your partner orders trump into your hand, you should at your first opportunity, show your partner what other trump you hold. Your partner, armed with this information, can then play the hand accordingly.
You're the dealer sitting in the South seat. You turn up the king of spades and your partner(N) orders it in into your hand.
West starts off the hand by leading the 10 of spades. Your partner(N) plays the right. East follows with the ace. Here is your opportunity to give your partner some information. As the trump ace has been played it means your king and the left are now equal. While from your hand it makes no difference what you which one you play. From your partner's viewpoint, things are completely different. Playing the king would signal to them you only have the trump you picked up. Playing the left would let him know you still have the king.