This theory asserts that the number of tricks available to both sides if they play in their best fit is equal to the total number of trumps in both sides’ best trump fit. Suppose there is a nine-card heart fit for North-South and an eight- card diamond fit for East-West.
This totals seventeen trumps. Therefore, if, on best play and best defence, North- South can make ten tricks in hearts, East-West should be able to take seven tricks in diamonds. With the same fits, if North-South could make nine tricks, East-West should make eight tricks and so on.
You can use this information in deciding how to bid on competitive auctions.
For example:
It is likely (though not certain) that the opponents have a nine-card heart fit. You expect partner to hold six diamonds for the three-level overcall, giving your side a ten-card diamond fit. With nineteen total trumps, you expect nineteen total tricks.
Therefore, if 4♥ is making ten tricks, you expect 5♦ to be down two. This tells you that it would be a mistake to bid 5♦ if you are vulnerable against not. You do not want to concede 500 to stop the opponents from making 420.
At other vulnerabilities, if you can make nine tricks (when 4♥ is making) or eleven (when 5♦ is), you are likely to show a profit by bidding 5♦.