North normally opened 1D. West either doubled or bid 1H and South would make some game-forcing bid, or maybe bid 3NT directly.

  

If East has bid hearts and South ended in 3NT, a heart lead from West defeated the contract, because Declarer cannot make a black-suit trick without letting the defence in.

  

In teams, after winning the HQ at trick 2, Declarer might try to sneak a trick by going to Dummy with the DA and playing the CJ. He is hoping to persuade East that he is intending to finesse, but he has little chance of succeeding.

  

But in pairs this is dangerous because you are likely to lose 6 more tricks very quickly. In pairs, you would cash your 7 diamond tricks and hope the Defence discard badly. (Note that because a low heart is led, West probably started with the HJ, hopefully with HJxx. Therefore it is better to win the second heart with the HQ, hoping that the blockage of the suit might be beneficial.)

  

If East had doubled and South ended in 3NT, it is normal for West to lead the SQ, and provided Declarer holds up the SK, 3NT will make.

Two pairs played in 6D, missing only 3 aces!!  I expect that, after South showed a strong hand with support for diamonds, North bid 4NT and interpreted the 5C response as 3 aces.

  

After doubling 6D, East had to pick a lead. Clearly there is a risk of the HA being ruffed, so East led the SA!! Now the HK can be discarded on the SK and 6D doubled is made.

   

East might have led a diamond to see if a view of Dummy helps. Since he holds aces in all side suits, this is not risky. And it works. When he is put in with the CA, he can lead the HA, safe in the knowledge that, if this is ruffed, partner, with HKJx remaining, can control the suit.