He would not, of course, have done this if he had known that the declarer held six spades.

It appeared that West held five diamonds, five spades and one heart.

East would not have doubled holding 13 spades, so he had to have an ace.

Now he had to hope that East held three spades.

It was likely that her partner held four spades, and if not he could correct to five clubs.

Holding up the ace was only going to be useful in the unlikely event that West held eight spades and no entry.

Notice that this plan would probably have failed if East had held three spades including the king.

If West had known that her partner held three spades and six clubs, she might have led a spade.

No, if West held three or more spades.

So South concentrated on the possibility that West held three spades, and found the way to succeed.