One of my best sources for deal, Victor Markowicz, played this deal online. As South, he held:

A9862
K85
AKQJ9
--

He opened 1 and his partner bid 2NT, Jacoby. Rather than describe, Victor decided to ask. He jumped to 5. That is the infamous "Exclusion Keycard Blackwood." It asked partner how many keycards he held, excluding the club suit. North's answer was disappointing: Zero! So, Victor put on the brakes in 5 and received a low club lead:

QJ105 J107 62 AQ96

A9862 K85 AKQJ9 --

Wanting to be in dummy, he took the A (finessing might result in East playing the K on the Q and no easy dummy entry). He led the Q and got good news and bad news. The finesse won, but West discarded a club.

Now what? The J held and then a spade to the 9 won the 4th trick (West continuing to throw clubs).

Should the last trump be drawn? This was the real deal:

	QJ105 J107 62 AQ96
-- Q43 108754 J8742		K743 A962 3 K1053
	A9862 K85 AKQJ9 --

If you draw the last trump, you are down. Diamonds misbehave and you have to lose a diamond trick. Eventually, you have to lose two more tricks.

At the key point (after the third round of spades), Victor saw that as long as diamonds behaved, he was fine. So, instead of drawing the last trump he started the diamonds from the top. East ruffed in and laid down the A, so the contract made easily.

East could have given declarer more of a problem (eventually needing a guess), but at least by keeping that trump in dummy, declarer was always able to survive the bad diamond break.