The traditional  opening lead against a NoTrump contract is fourth from the top card in leader’s longest suit. Subtract that number from 11 to establish the number of higher cards in the other three hands.

If the leader plays the ♥️4, there are seven cards higher in the other three hands. Let’s say the dummy has been tabled. Here are your hearts and dummy’s:

    DUMMY
    ♥️ 10 6

    YOU
    ♥️ Q J 5

With five cards higher than the ♥️4 in your hand and dummy’s, you know that your right-hand opponent holds two. Sometimes observing the Rule of 11  —  and it’s a genuine rule, not a guideline masquerading as a rule — can be especially rewarding. Let’s say the ♥️7 has been led. You and Dummy hold:

    DUMMY
    ♥️ Q 10 9

    YOU
    ♥️ 8 6 4 3

With four cards higher than the ♥️7 in the visible hands, you shout “Eureka”! RHO holds nothing higher and you can win with the ♥️8.

Let’s see how quickly you learn. Here’s a hand from a column written by the nonpareil bridge writer Eddie Kantar. South is dealing, with no one vulnerable:

                                    NORTH
                                   ♠️ A K J 2
                                   ♥️ K 6 3
                                   ♦️ 7 6 2
                                   ♣️ 6 3 2

WEST                                                      EAST
♠️ Q 10 7 6 3                                         ♠️ 5
♥️ 10 5 4 2                                             ♥️ Q J 7
♦️ J 10                                                   ♦️ Q 9 4 3
♣️ 10 4                                                  ♣️  K Q J 9 8
   
                                   SOUTH
                                  ♠️ 9 8 4
                                  ♥️ A 9 8
                                  ♦️A K 8 5
                                  ♣️ A 7 5

South  West  North  East

1NT  Pass  3NT  All Pass

West leads the ♠️6. What do you do?

Simple: subtract West’s 6 from 11. The answer is 5. And how many cards higher than 5 are in the North and South hands? Five! So you can win the opening lead with your ♠️9 and take the marked finesse to the ♠️J. Your four spade tricks, plus the five sure tricks in the other suits, provide the nine tricks you need to make the contract.

If I lead a low card against a suit contract, I’ll also use the Rule of 11.

Would you believe there are bids that show precisely 11 high-card points? Partner bids 1♣️ and RHO overcalls 1♠️.

You hold:

♠️ A K Q
♥️ Q 6 5
♦️10 9 8
♣️ 7 5 4 3

A bid of 2NT shows at least one spade control and 11 HCP. With 10 HCP, you should  bid 1NT. With 12 HCP, bid 3NT under the modern philosophy that 12 + 12 = 3NT.
Here’s another 11-pointer that can be bid precisely. You’re  sitting North, with South dealing:

♠️ Q 10 9 8
♥️ K 4 3
♦️ 5 4
♣️ A Q 6 4

The bidding has proceeded as follows:

South              West           North           East
1♦️                  Pass           1♠️               Pass
1NT                 Pass           ?

By all means, bid 2NT, showing 11 HCP and telling opener to bid 3NT with 14 HCP or 13 HCP with a running five-card suit.

We know that 7 is a lucky number. Don’t discount 11!