Most players won't know that they have erred when they cash a top heart first, because it does not matter 90% of the time. A 2-2 break is 40% and a 3-1 is 50%. As you are only in 6
♥, you can afford to lose a trick (3-1 with the queen among the three).
However, when everything looks rosy it is time to take stock and ask yourself: "is there anything that can go wrong?" Well, yes and no. A 4-0 break (10%) is most annoying. "Is there anything I can do if they break badly?" In this case the answer is "yes".
If you cash a top heart first, you must lose two tricks, one to the queen and one to the 10. Similarly ...
East must come to two tricks if you cash the ace or king first.
Now, try the effect of leading a low card towards dummy's J5. If West has them all, he can score his queen now or later but he won't get more than one trick. You still have A, K andf J left to draw his 1083.
If East has all four, West will show out when you lead low from your hand, and you play dummy's jack. East can win his queen, but he won't get another trick because you now have AK9 over his 1083 and still the 5 left in the dummy. Next time you play the 5 and cover East's card as cheaply as possible.
This is called a safety play, this time a play that guarantees your contract even if trumps break 4-0. Many times safety plays are line of plays that are not 100% but that improve your chances in case suits break badly.
Our "expert" should definitely have read a book on safety play. Maybe he will now after he went down in a slam he could and should have made. At any rate, he may not yell "Why not 7???" if this comes up again.
Roland