Lets talk about the following hand, might be a good lesson:
Suppose you are sitting in East. Your opponents had a bit of an agressive auction:
pass - pass - 1♦ - pass
1♥ - pass - 3♠ - pass
4♦ - pass - 4♠ - pass
5♥ - pass - 6♥ - Dbl
Partner leads ♠K, dummy ruffs with ♥4 and plays ♥Q.
First question: do you take your Ace or not? There's no real reason to take it. Playing low gives away a trick (double dummy), but you make it very hard on declarer to get to 11 tricks, since he can't draw another trump! I think most good players would hold it up once.
At the table East cashed ♥A, so we cash and have another decision to make.
The key now is to figure out what declarer needs to make his contract, and what dangers he'll have to cope with. It's clear that he'll need his ♦ length (which you control), and he'll also need to draw trumps at some time without us getting another chance to play some ♠s. To set up his ♦s, declarer will need ♣ entries (you only have 2) and 2 trumps to ruff. So it might be possible to lock up the hands.
There's another question that is probably very interesting: how many ♦s does declarer have? We can't know for sure, but it's probably 2-3 since most players don't cuebid partner's suit with a singleton. If he has 3, then you can probably give partner a ruff, but in that case you'll always have a ♦ trick so there's no need to play ♦. If he does have a singleton or void, he'll be totally ruined anyway (with stiff he needs to ruff 3 ♦s, so he'll need a 5 card trump).
So what do you continue?
It's necessary that you mess up declarer's plan. You suppose declarer has ♦Kx (which is the only dangerous one for us). He will be able to set up his ♦s only if he can draw your last trump when he's in dummy (since he'll need his other trumps to ruff ♦s), ready to cash his remaining ♦s. So there are 2 possibilities to mess that up:
- let declarer ruff a ♠: declarer will have to hope for a 3-3 split in ♦ now, which isn't the case (he'll need to draw trumps, and will only be able to ruff 1 more ♦, not 2)
- mess with the ♣ entries. You only have 2, you declarer will need 2 ♣s to get to dummy twice. Now that you'll ruff the 3rd ♣ he'll be down.
At the table, East played a ♥, which is imo the least useful continuation possible. Now my partner (I was sitting North) found a very nice line of play: ♦K, ♦A, ruff a ♦, ♣ to dummy, ruff another ♦, ♣ to dummy, draw the last ♥ and claim for contract.
If you visualize declarer's plan, you'll have 'double dummy vision' playing this hand.