Several Lessons

A 6 3
A Q 10 7 5
J 10 8
10 2
K 5 4 J 10 9 8 2
8 6 4 3 2 K J 9
6 5 3 7 2
K Q J 5 3
Q 7
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A K Q 9 4 Opening Lead: 6 of hearts
A 9 8 7 6 4 Vul: N-S.

SouthWestNorthEast
1 Pass 1 Pass
2 Pass 2 Pass
3 Pass 4 pass
5 pass 6 All pass

This hand has a couple of lessons -- well, three really, which I'll get to by and by. First is the value of the more balanced suit as trump. One of the reasons given for that is pointed out here, namely with a balanced suit for trump, you can often ruff out a round of the unbalanced suit which you couldn't do if you named it trump. Here it is evident that in diamonds at the six level, declarer could afford to lose a club and ruff the third round with impunity, regardless of the defense, which could get in two trump leads, but not inhibit the ruff. Declarer would then be on claim.
The second lesson is preference bids. Declarer bid clubs and then reversed in diamonds. He might have clarified his hand a little if he'd rebid diamonds to show five on his third bid. Bidding the lower ranking suit first and then the higher ranking twice should show exactly the distrubtion South has. Nevertheless, it was incumbent on North to show preference, to show that he had more diamonds than clubs on his third bid, which was the first one where he bid one of his partner's suits. He should do it quickly here. If declarer now rebids clubs, it would indicate, I would say, a 6-4 holding, saying, I hear your preference, but I fear a 4-3 trump suit. Do you have a couple of clubs for me? Or four diamonds?
You can find a discussion of preference bids here. There I have said that when the choice is between two majors or between two minors, I play it right down the middle, which is to prefer the suit where you have the longer holding, and if of equal length, prefer the first bid. Yes, I suppose I could "violate" that principle if after I'd shown preference for the second suit, my partner reverted to the first, which I would take to mean a 6-4 holding, as just mentioned in the above paragraph.
But declarer should be happy with a diamond preference if only his partner had made it and carry on to the best spot, which is 6 diamonds. In clubs you've gotta lose two trump tricks. Or do you?
One declarer didn't lose two trump tricks in clubs! That was effected by East's cover of the 10 of clubs! In an unbid suit, I would consider that a reasonable cover. Whatever your partner holds, it's unlikely that a simple cover will bump into anything he holds, and it could promote a trick. But here, the suit was not only deemed worthy of being named trump, but declarer bid it three times!
Please bear in mind that when you cover, you do so to promote a lower card, which here would mean that you cover a 10 to promote a 9 or perhaps an 8. You don't cover to promote a queen, since it can already beat a 10 and the same holds for a king. So if your partner holds 2 clubs, you can't promote the 9. If he holds 3 clubs? Yes, there are two holdings where that would do the trick, to wit, your partner holds A 9 low and K 9 low. The cover would indeed beat the slam. Incidentally, a queen doubleton by West would be another way where the cover loses a trick, though here that would only be an overtrick lost. In any event, length by declarer, and hence, a short holding by your partner is one reason for being very cautious about a cover.
But you not only have to place your partner with 3 clubs, which is questionable on the bidding -- remember, South reverted to a rebid of clubs before a preference by his partner -- but if by chance he does hold 3 clubs, that's an awfully specific holding for him to have to warrant the cover. Reasons for not covering an honor are discussed here.
In any event, it should be evident that declarer should be very safe in 6 diamonds: lose a club, cash the ace on regaining the lead, ruff a club high, of course, run trump and claim. North cannot know that the slam contract hinges on just that situation. What he does know is that he has more diamonds than clubs and should grant his partner the courtesy of preferring the suit where he's longest.