Here was the full deal:
Partner didn't double and I chose a diamond on lead. Of course I might have guessed a heart, but I didn't see anything obvious to suggest a lead with my hand.
Of course our teammates were very understanding that we didn't defeat the slam. It was just an interesting question over how risky the lightner double was.
What we came up with is that West has shown one ace. The lightner double gambles that the ace is NOT in spades. I.e. whenever East has the spade ace, you want a heart lead. I'm not sure if that the analysis is complete, as if West has the
♥A we can certainly come up with layouts where slam is cold as well. However, you risk about 5 imps if it makes (unless they rewind), where as you stand to gain much more than that.
1♠ - 2♥(1)
2♠(2) - 3♦
3NT - 4NT(3)
5♦(4) - 6NT