Okay, here's the full deal.

     Dealer:  South
     Vulnerable:  North-South
 
                             North
                             S:  9
                             H:  Q J 10 9 3
                             D:  J 10 6 3
                             C:  9 8 2
 
     West                                         East
     S:  10 7 6                                   S:  Q 8 2
     H:  6                                        H:  A 8 7 4
     D:  Q 7 5 2                                  D:  K 9 8 4
     C:  A Q 10 7 3                               C:  K 6
 
                             South
                             S:  A K J 5 4 3
                             H:  K 5 2
                             D:  A
                             C:  J 5 4
 
     Opening Lead:  6 of hearts

And here's the auction (repeated for convenience).

     North    East     South    West
                        1S      Pass
     Pass      1N       2S      Pass
     Pass     Pass

As the missing cards lie, returning a small club to East's king at trick 3 lets East lead another heart for another ruff, also cutting off declarer's communication with dummy. West can then cash the ace and queen of clubs, and East later wins a trick with the trump queen for a set of two tricks. Any other lead concedes the contract, with overtricks.

>> On a diamond lead to trick three, South's ace wins the trick. South then cashes the ace and king of spades, stripping the rest of West's trumps and leaving East with the queen. South then plays his last heart to dummy's queen-jack-ten, sluffing two of his clubs regardless of whether East ruffs or not. East-West win tricks with the queen of spades and a club honor either way, but declarer gets an overtrick either way.

>> And a spade lead to trick three is even worse, as it autofinesses East's queen. Declarer takes the three trump honors, pulling all of the outstanding trumps, then run dummy's hearts, sluffing two clubs. East-West win a trick with a club honor, but declarer's remaining trumps yield a pair of overtricks.

Of course, the result would be very different if one swaps South's ace of diamonds and four of clubs with East's king of clubs and four of diamonds.

Ultimately, this comes down to a question of whether the East-west pair interpret the return of the four of hearts at trick two as a signal indicating a club entry or not -- and that may well be the difference between highly ranked intermediate players and the true experts. Nonetheless, I don't see any reason why East should know to prefer a club return over a diamond return when leading to the second trick.

BTW, this is a rotation of the deal in a previous thread "On Defense..." (http://boards.fool.com/on-defense-30255885.aspx for initial post).

Norm.