Major Pain

Only South can make slam in spades, clubs, and notrump. If East is on lead against these contracts, then a heart lead sets up the heart king, and in each contract, declarer must lose a trick in spades or clubs to make the slam, at which point, the defense could take their heart trick.

On the other hand, the heart slam only makes when declared by North.

The problem is that South's hand is very shy of entries.

If South declares 6 ♥, west leads a spade. Declarer finesses the queen, cashes dummy's club ace, then leads a trump to the ten.

West wins the king, and continues spades, and now declarer is tangled up. He'd like to ruff a small club in dummy, but there is only one entry to his hand.

But why is this spade lead better than a passive minor lead?

If West leads a minor, then the position ends up:

Now oddly, the additional entry to North's hand makes all the difference.

Declarer plays the queen and ace of hearts, then ruffs a club in north, plays the ace of diamonds and overtakes the queen and runs clubs leading to this position:

West is caught in a positional squeeze.

Obviously, if North is declaring 6 ♥ the spade lead does not work, because it blows up the suit.