Being aware of your opponents defensive signals can be useful in many ways. One can watch their opponent's signals to learn where high cards are placed. Sometimes by watching their Count signals you might find the missing piece to the puzzle of their hand. Occasionally your opponents will play special conventions like Smith. If they do, it is imperative to understand the method, regardless if you choose to play it yourself.
Smith is a type of defensive signal most commonly played only against NT.
The defense often will have trouble knowing if it is safe or correct to continue the suit they started with on opening lead. There may be a different suit that is imperative to switch to. After declarer wins the opening lead suit at trick one. Rather than just playing their cards in random order, some defenders attempt to give their partner a signal about whether they liked the opening lead. When playing regular Smith the defense has two options of which way to play their otherwise sometimes irrelevant spot cards.
They can play a higher card first (eventually followed up with a lower) to say they LIKE the opening lead. Examples:
1) You have something partner doesn't know about yet in the opening lead suit (for instance you played the J 3rd hand high, and the declarer wins the king. The opening leader would normally have to guess who has the Q.
2) You think that the best possible defense would be to continue the opening lead suit
Playing a low card first (eventually followed up with a higher) would say they DON'T LIKE the opening lead. Examples:
1) You do not have whatever help that partner may need in the opening lead suit
2) You have another suit that you want to start attacking so partner will be turned off continuing the lead.
On this deal today as declarer, aware my opponents were playing smith I chose to play a specific suit first to get as much information as possible.
Important note from bidding: 1NT promised 7-10, with a worse hand partner would just choose one of the three suits
When my opponent lead a fourth best spade I flew Jack and saw the 9 on my right. I learned at trick one that lefty had exactly 5♠ because they lead 4th best and it was the 2nd lowest spot out there. My left hand opponent's spades were quite good... ♠AQTxx. They clearly intended to tell their partner to continue spades. I chose to cash a high club at trick two. My opponents are very rarely going to lie with their smith signal because its very difficult to know exactly when partner won't need the information. Since my right hand opponent likely didn't want spades back, I assumed that the ace would draw their lowest club (in the actual hand they played the 7 on my right and the 8 on my left). My right hand opponent was essentially forced into playing his lowest club and that helped expose to me that my left hand opponent, whom I knew liked spades, told his partner he didn't like spades.
Since the low club on my left was obviously not a true signal, it could only mean that he has a combination in the suit which he was unable to give the proper signal. That narrowed it down to 1 of 2 holdings. ♣7 or ♣Q7. Now that you have this information, play your line of play...
Plus... it's free!