Imagine this time that your partner opens 1♠, they pass to your right. Your hand:
If you count properly, you have 10 HCP (4 H + 2 D for the 9th trump + 1 D for the 10th trump + 2 D for the singleton Hearts + 1 D for the doubleton Clubs). So normally, you would bid 2♠ since that promises 6 to 10 HCP.
But hey, you might feel that you will probably make a lot of ruffs, so that you can easily make 4♠. And especially, since you don't have many HCP, the opponents might also make a contract!
By the way, look at the 4 hands that day:
If your side plays 4♠, you lose 2 tricks in Diamonds and 1 in Clubs. Just made.
If the opponents play 5♣, they lose one trick in Hearts and one ruff if you are inspired enough to play the Ace of Hearts and Hearts ruff (not simple).
Note that they don't lose any Diamonds by making the finesse to the Queen (the ♦4 towards the ♦J, the honor that is not master but you are a pro at finesses now).
In short, by looking at a large number of deals, a guy named Jean-René Vernes found a law whose practical formulation is the 'Law of Trumps'.
Partner has promised five Hearts. With a weak and distributed hand, you hesitate between level 3 and level 4 to preempt. You also have five Hearts. Which level corresponds to the known number of trumps?
Section quizIt means that even if you go down, they will generally score less than if they had played a contract on their side. Attention, it's statistical: I don't take complaints if it doesn't work on a specific deal!
A pair preempted at 3♥ with nine trumps and went down one trick. Is this enough to conclude that their preempt was a bad decision?
Section quizSo with the hand I gave you, it was well played to jump to 4♠ over 1♠, just to bother them as much as possible.
Come on, it's your turn to join the action!