You pick up
Matchpoints. You have 11 HCP. Not enough to open, and anyway Righty opens a 15-17 1NT. You pass resignedly, Lefty bids Stayman, Righty shows hearts, and Lefty jumps to game (showing by implication a 4-card spade suit).
Time to count points. The opponents bid game, so they probably have at least 25 HCP. You have 11, so you can reasonable hope for your partner have 4. So partner will be some help on this hand, but not a lot.
Because partner doesn't have much, you might not want to lead away from an honor. But a heart lead is liable to finesse partner (when declarer would naturally finesse you without your help). So you decide to lead a diamond -- either the 10 or 9, depending on how you play.
Dummy comes down with
Declarer plays the Q and it wins, partner playing the 2. That could be the two from J2 of diamonds. Otherwise, partner doesn't have the jack of diamonds.
Declarer leads a club from his hand and thinks, presumably about finessing. He decides not to and plays the K. This hand has been nothing but bad news -- your opening lead didn't work, dummy came down with a solid 10 HCP, and now declarer didn't finesse into your Q, he is probably going to finesse into dummy.
Declarer changes tack (which now that I think about it, is a little strange). He cashes 4 rounds of hearts, playing the KJ of hearts from dummy, then the AQ from his hand. You have to find two discards and opt for two diamonds. Declarer pitches a club from the board, and partner follows to four rounds of hearts in informationless unconcern.
Declarer now leads the J. There is some chance that declarer does not have the 10, and no chance that declarer will not finesse if declarer does have the 10, so you cover the J with your Q. Declarer now leads a diamond from the board and finesses the J.
What can you count?
No one has shown out of anything but hearts. You can know that declarer had at least 3 diamonds, at least two clubs, and 2-3 spades. That doesn't add up to much -- in fact, you have somehow muddled your way to trick 8 without knowing anything more about declarer's distribution than that declarer started with 4 hearts. And you knew that from the bidding.
Declarer has shown the AQ of hearts, the AJ of diamonds, and the KJ of clubs. That's 15 HCP. You have hit the information jackpot! You have seen all of the high cards in every suit but spades. Declarer could have the J, but declarer is unlikely to have the K, because that would give declarer 18 HCP.
Without counting, your natural instinct might have been to exit in diamonds and wait for your two spade tricks. Now you know to attack spades.
A. Ignore what partner plays. Q, declarer playing the jack. 9. Declarer thinks about it and does not play the 10 from dummy, so your partner cannot overtake with his K. But you lead a club, and yes!, partner has the 10 and cashes the last two tricks. Down 2, which earns 12 of 12 matchpoints.
(Setting it just one would have been 11.5 out of 12. If you return a diamond, declarer scores two diamond tricks and makes the contract, but you still get 7.5 out of 12 match points.)