A Story Hand #Two

West leads the two top hearts and sees East's high diamond. West switches to a low diamond and East returns the suit. West now goes back to hearts. How is South supposed to work out what to do?

Might not South ruff with the king of spades and then finesse East for the jack? That would be necessary if East started with the J76 of spades. Given that East has only one heart, South reasonably might decide that West has just one spade. If he does this, down he goes. A mystery trick from nowhere.