Hands are from Adventures in Bridge, September 26, 2018.
South: ♠875 ♥AQ107 ♦AQJ942 ♣—
North: ♠AJ93 ♥K865 ♦K63 ♣A2
This hand was a good example of the importance of distribution in effective bidding. South opened 1♦ and North bid 1♥. South jumped to 3♥. [Theoretically South could jump to 4♥ as South has only 5 losers by Losing Trick Count in hearts, but one should not stretch LTC to the max. Partner could easily have bad hearts, good clubs and poor spades.]
North was highly encouraged and used Roman Key Card to find two Key Cards with the Queen in the South hand. North did not bother to ask for Kings for two reasons: (1) even if South were to show two Kings, 7♥ in unbiddable because you cannot count 13 tricks; (2) North heart's suit is mediocre, so a 4-1 heart break could doom a grand slam if partner does not have the ♥J. So, North simply bid 6♥. When hearts divided 3-2 and diamonds were 2-2, making 13 tricks were easy.
Only 6 out of 16 pairs bid slam. One pair did bid the grand slam; the other five bid 6♥.
East (Dummy): ♠A95 ♥Q852 ♦AJ8 ♣Q102
South: ♠K74 ♥1096 ♦9532 ♣AK7
The opponents have reached 4♠, and your partner leads the ♣4. Declarer calls for a low club and you are forced to win the King. Declarer follows with the ♣5.
This hand is actually a problem defensive hand, playing matchpoints. You are sure that you have three tricks: the ♠K behind the ♠A in Dummy, and two club tricks. (Partner has led fourth best, so has either four or five clubs to the ♣J, and Declarer has either the doubleton ♣9x or ♣9xx.)
You know there is no future in diamonds because any diamond finesse needed is going to work for Declarer. Partner might have a heart card. If partner has the ♥A, it cannot go away since you can play a heart immediately after cashing a second club. If, however, partner has the ♥K, you need to play a heart NOW—at trick two—before cashing a second club because the second club will set up the ♣Q in Dummy for a discard if partner started with five clubs.
On the other hand, if Declarer has both the ♥A AND the ♥K, your second club trick could go away on the heart suit if you switch to hearts at trick two. Declarer is known (from the bidding) to have an opening hand and six spades.
So if Declarer has ♠QJ10xxx ♥Ax ♦KQ4 ♣95, you need to switch to a heart immediately.
If Declarer has ♠QJ10xxx ♥AK ♦Q10x ♣9x, a heart switch will give Declarer an overtrick and destroy the value of your partner's best opening lead.
If Declarer has ♠QJ10xxx ♥AK ♦Kxx ♣9x, a heart switch will again by deadly for your side.
I played it safe and cashed my second club before leading a heart. That was wrong, because Declarer had ♥Ax of hearts and partner did have the ♥K. C’est la vie. At IMPS, the heart at trick two would be mandatory.
Of the 16 tables, 9 pairs bid and made 4♠; 3 pairs played in notrump (making 3 and 4); 3 pairs went down in 4♠ (all with the killing club lead), and one outlier was in 6NT, down only one!