Deal 19
 
  6♠
OL: ♠5
♠ K 9 8 6 4
A 6
Q J 3
♣ A K 9
 
 WEST  NORTH  EAST  SOUTH
1NT
pass2pass2♠
pass5NTpass6♠
passpasspass

You are East. The bidding has been as shown.
The 2 bid was a transfer.
The 5NT bid meant to choose either 6♠ or 6NT. South liked the suit.
Partner led the ♠5, won by South's ♠Q.
South pulls one more trump with his ♠A, then wins three high s.
Next comes dummy's A on which he plays the 3 and West plays the 9.
Then he leads dummy's 6 which you win with the Q.
On this trick South drops the J and West the 2. It is your play.

When you have made your choice click NEXT .
♠ 5 7
9 2
2 6 8
♠ 10 3
K Q 8 5
9 7 5 4
♣ Q 8 4
  ♠ Q A
3 J
A K 10
 

 
Deal 19
 
  6♠
OL: ♠5
♠ K 9 8 6 4
A 6
Q J 3
♣ A K 9
  Play the K.

If you believe South is now out of s then you shoud not play your K because it would give him a ruff-sluff.

But you shouldn't believe South when your partner has sent you a clear signal that he holds an EVEN number of s.
In this case an even number must be 4 which means that South has one more .
So you play your K. He ruffs in dummy of course, but then has no way to get rid of that ♣ loser when the finesse fails.

If you had not been paying attention to West's signal you might have decided to risk a ♣ lead - with a disastrous result.

 Deal 20 
♠ 7 5
9 7 4 2
8 6 2
♣ 7 6 5 2
♠ 10 3
K Q 8 5
9 7 5 4
♣ Q 8 4
  ♠ A Q J 2
J 10 3
A K 10
♣ J 10 3