LEARNING CENTER

Switch to a spade. Don't let declarer ruff a heart loser. Partner led  the 3 of hearts which means declarer has at least 3 hearts (partner doesn't have five of them or would have led differently). If you continue hearts, declarer can ruff the third round in dummy. There's no risk of declarer discarding all of their heart losers on anything in dummy with your decent diamonds. 

The full deal: 

Vul:None Dlr: South	43  Q5  Q7643  K832
K10  10873  A2  A9754		765  AKJ2  J1098  Q6
	AQJ982  964  K5  J10

Overtake and continue hearts. Partner started with two hearts (or maybe one). Here, if you continue hearts, declarer ruffs with the hand that has more trumps (dummy). This is sometimes called "tapping", since you are draining their long trumps. It is VERY different than allowing declarer to ruff with the hand that has fewer trumps. If you force declarer to ruff with the North hand, he isn't getting anything "extra". Take a look at the full deal and see how declarer is going to have a tough time. If you played diamonds or clubs, you'd make declarer's job easier. 

The full deal: 

Vul: None Dlr: South	K10532  83  J76  J74
QJ94  J2  102  Q10853		76  AKQ109  Q985  K6
	A8  7654  AK43  A92

Spade. Your partner is saying "I don't like hearts" by playing his lowest heart. Playing a diamond or a club would be dangerous. A spade is safe. Whether or not partner has the K, you aren't giving anything to declarer. See what happens if you switch to a minor (or continue hearts) on the full deal. Declarer may go down two against perfect defense (losing one spade, two hearts a diamond and a club), but comes close to making if you misdefend. 

The full deal: 

Vul:None Dlr: South	AQ65  754  Q3  Q987
432  KQ109  K865  J5		K9  632  109742  K43
	J1087  AJ8  AJ  A1062

Switch to a club. Partner might have heart honors, but so what? The diamond suit is very dangerous looking from your perspective. If declarer has the K, then they'll have five diamond tricks. If not, then the K is finessable and the suit is splitting evenly. This is the time to get aggressive with a club switch, hoping to find partner with something in clubs. 

The full deal: 

Vul: None Dlr: North	KQ52  9  AQJ87  532
103  10873  K93  AQ76		987  AJ65  654  J109
	AJ64  KQ42  102  K84

A low spade. Why? If you play the ace and another spade, declarer can still manage to ruff a heart because partner can not draw dummy's third round of trump. If you play a low spade, partner can continue spades whenever they get in. On a good day maybe partner has Kx or KJ10 of spades, and he can draw all of dummy's trump right now. A lot of different hand types are possible for South, but playing a trump follows the golden rule: don't let declarer ruff in the hand with fewer trump. Here's one possible layout: 

Vul:None Dlr: W	Q87  102  KQ76  K432
62  KJ953  A10  AJ109		A43  A87  J943  865
	KJ1095  Q64  852  Q7