Opening Leads Against No-trump Contracts
First, let's consider what you want to do in an uncontested auction. That is an auction when you and your partner have only passed. A typical auction would be:
You know East has 15–17 HCP, and West has about 10 HCP, maybe a few more. For the purpose of playing defense with this type of auction, figure that the opener has 16 HCP, and the responder has 10 HCP. Your first consideration will be to calculate how many HCP are remaining for your partner to hold.
Your partner has about 8 HCP. You know this because you add the number of known HCP in each of the hands. The opponents have a combined 26 HCP, you have 6 HCP, and the remaining HCP are in your partner's hand. It's only simple arithmetic and you can do it.
When firing the opening salvo against a no-trump contract, lead the fourth-highest card from your longest suit. Although not a high card, it's an aggressive lead. Your goal is to win control of the suit led by retaining high cards in that suit. The logic is that you're trying to retain your high cards for later. When you lead your fourth-highest card, your partner will know your best suit.
In this type of auction, it is usually best to lead from your longest suit. You will consider two questions. First, what suit, and secondly, which card? A general rule when leading against a no-trump contract is to lead your fourth-best card from your longest suit. With this hand, you have two five-card suits, both spades and diamonds. Which one to choose?
Neither of the opponents has bid a major suit. West has not used the Stayman convention asking the 1NT opener for a major suit. The suggestion is that the opponents are weak in one of the two major suits. The only hope from your perspective is that the opponents' weakness is in spades. You will lead the fourth-highest card from your spade suit, specifically, the ♠ 6.
With this hand, you hold 5 HCP. Your partner should have about 9 HCP. Your only information from the auction is that your opponents have not tried to find a game contract in a major suit by using the Stayman convention. Your best bet is to lead the fourth-highest card from your major suit. Specifically, lead the ♥ 4.
Here is your hand:
Your partner has very few points. You can do the arithmetic and give your partner about 1 HCP, maybe none at all. You will probably have to win all the tricks for your side. You will start with a very aggressive lead. To figure out what you want to lead, first consider which suit you do not want to lead.
You do not want to lead a singleton against no-trump. You can eliminate the spade suit from consideration. Your club holding is very nice and will win two tricks. Save those for later and eliminate that suit from consideration. You are left with hearts or diamonds.
If you lead a diamond you would have to force out the ace, queen, and jack to win an extra trick. On the other hand, if the declarer leads the diamond suit first, she will have to allow you to win the ♦ K and maybe even the ♦ 10 before she can run the suit. She will have to give you the lead back and you can continue to attack the heart suit.