Deal 12
 
  4
♠ J 3
K Q J 6
A K Q 10 5
♣ 6 2
 
 WEST  NORTH  EAST  SOUTH
1pass1
1♠4passpass
pass

The bidding has been as shown.
Partner led the ♠A, then the ♠K.
Dummy follows suit and it is your play to the second trick.

When you have made your choice click NEXT .
♠ A K


♠ 10
7 3 2
9 8 6 3
♣ Q 7 5 4 3
 


 

 
Deal 12
 
  4
♠ J 3
K Q J 6
A K Q 10 5
♣ 6 2
  Ruff partner's ♠K.

It is obvious that your side can only get 2 ♠ tricks.
And assuming that South has the Ace of trumps you will not make a single trick in the red suits.
So you need to make 2 ♣ tricks.

If partner has the ♣ A K then he will take them, but if he has the ♣ A J he cannot win 2 ♣ tricks from his side.

To allow for this situation it will be better to play ♣s from your side of the table, and the way to make that happen is for you to ruff partner's ♠K with an otherwise useless trump. You still get your two ♠ winners (sort of) but now you get to play a ♣ through South's ♣K at trick 3.

 Deal 13 
♠ A K Q 8 6 4
8
7 4 2
♣ A J 9
♠ 10
7 3 2
9 8 6 3
♣ Q 7 5 4 3
  ♠ 9 7 5 2
A 10 9 5 4
J
♣ K 10 8