The Devil's in the Detail!!!

This is the last deal in our weekly online training on 12th Mar, 2018.

Board 16
Vul: E-W
Dealer: West

North      East     South     West
                                              1♢
 Pass        2NT      AP

Lead: ♠6




Trick     North     East     South     West
   1         ♠Q         ♠K        ♠6           ♠2
   2         ♢5          ♢2        ♢4           ♢A
   3         ♢6          ♢3        ♠3           ♢K
   4         ♢Q         ♢8        ♡3           ♢9
   5           ?

Lead: ♠6

The declarer wins with the ♠K and runs two top ♢. You win with the ♢Q in the third round. Your partner discards ♠3 and ♡3. And now what's your plan?

The declarer could win 4♢, 2♠ and 2♡. ♠3 shows that your partner holds 5♠. Assuming that you return ♠ now, even if your partner holds ♠A, the declarer could return back twice easily to finesse ♡. In other words, discarding small ♡ and ♠ indicate that your partner doesn't hope you to return ♠. So shifting to ♣ is the only choice. To a player with certain experience, shifting to ♣ is obvious. And now the problem comes, which one to play? Will you play 5 casually? You just have 2 entry to through ♣. Assuming that you play ♣5, it does show that you hold 4♣ with top, unless the declarer holds both 3 and 4, you need 3 entries for follow-on defense. And it might encourage your partner to continue ♣, although after deep thought, your partner should not continue with ♣. You do dig a hole for your partner. For follow-on defense, ♣9 is a better choice. After winning, your partner will choose ♡ or ♠ according to his cards.

PS: If you return with ♣5, this deal becomes a playing problem. Here I will not go further on this issue.

The full deal:
Board 16
Vul: E-W
Dealer: West