NORTH
Q543
65
93
QJT65
9876
AJT98
7
A97
AK2
K32
KQ2
K432
  JT
Q74
AJT8654
8
Dealer: West    Vul: None   MP Scoring

West
Pass
X

North
Pass
Pass

East
1
Pass

South
3
Pass

 Counting Downtricks 
This board rotated to make declarer South: original board number: 1

East had a tough choice in response to West's negative double, which showed 8+ cards in the two unbid suits and about 10 points.

The A was led, winning trick one, East signalling an even number of clubs with the 'high' 4.
East's 1 opener promised at least 3 clubs. With 5 originally in dummy and West seeing originally 3, East was marked with precisely 4 clubs. That meant to West that East held fewer than 4 diamonds, at most 3.
With a 4 card major, East would have almost certainly have bid game in that major.

East didn't have as much information after trick one about the West hand.
If declarer could be depended upon for 7 diamonds, West would have exactly one (13 - 7declarer - 3 East - 2 dummy = 1).

West switched to the 9, tending to deny both the JT. East won K then A.
The K was tried, declarer ruffing.
A small heart was led by declarer, West winning the 8.
West thought that East had started with doubleton spade and tried to give a ruff.
Declarer won in dummy, discarding a heart, then discarded another heart on a club.
Trumps were drawn and declarer got out for down one.

East insisted that with doubleton AK, first cashed would have been the A.
East had two opportunities to lead trumps and West had three opportunities to lead trump.
If declarer could have been held to 6 diamonds alone, down 3 would haver resulted in a very good score for East West indeed.

   1>     6                       6     6     2-    2-   10-    8-    2-    8-    2-   10-    0 
430 ---- ---- ---- 430 430 420 420 460 450 420 450 420 460 100
If East had counted 18 in hand, 9+ for West, 5 in dummy and the declarer's A as 4, 36 points were accounted for.
If Declarer had held the A, the J would have to be with West, as would the QJ.
Many things suggested that after winning the K, East lead a small diamond to prevent declarer from ruffing a heart in dummy.