This deal was played in "the local duplicate" in Boca Raton earlier this year. I have many teaching deals in my arsenals designed to locate a queen. Finding a jack is a much more difficult topic, so I was delighted when David Berkowitz called to tell me about this deal he played at the club. He held:
 K108743
 K2
 Q65
 J2

After three passes, nobody vulnerable, he opened 2. In 4th seat the range for a "weak" two bid is about 9/10 to 13/14 HCP (with a really weak hand, you'd pass the deal out). His 2 bought the contract and the 10 was led:

Q2  Q954  J72  KQ109

K108743  K2  Q65  J2

East cashed two high diamonds and played a third round, all following. How will you play the trump suit? That is sort of a trick question. The normal play in spades is low to the queen and later to finesse the 10. However, David did some exploratory preparation.

He led a club to trick 4. LHO won the ace and cashed the A and played another heart to declarer's king. Any new ideas about the spade suit?

Remarkably, you can place both the ace and the jack! West won't have the A. Why not? He already has the A and A. Do you know any player this century who would pass with 3 aces? Not me. So, East has the A. He also had the AK. Personally, I'd open with AK-A, especially in 3rd seat. This East player was conservative, yes, but if you add in the J, I'll bet anyone would have opened.  That was David's conclusion. East had to have the A (West couldn't have 3 aces) and West had to have the J (East would have opened in 3rd seat with 12 HCP including AK-A).

Backing his convictions, he led the 10. If West had J9x, nothing could be done. If West had J9 doubleton, David would likely misguess the suit (10-jack-queen-ace and later a finesse against the 9, better odds than J9 doubleton). However, this was the real deal:

Vul:East-West Dlr: West	Q2  Q954  J72  KQ109
J65  AJ7  10983  A54		A9  10863  AK4  8763
	K108743  K2  Q65  J2

West actually didn't cover the 10 (would you?) for fear declarer had K109 and was trying to get a cover. Anyway, David backed his card-reading and let the 10 run. Making 2!

This is the kind of deal where "normal" players would lose 2 spade tricks by the very normal play of a spade to the queen and ace and then an odds-on losing finesse. They would later see the double-dummy indication that it makes--and realize that running the 10 was the winning play. "Not findable," they'd ruminate. Unless you are a hall-of-famer.