South rises with the ♦A and returns the ♦7, the higher of two remaining cards. With three remaining cards the lowest is returned. West plays the ♦9 and North wins with the ♦10. North knows, or should know, from the return of the ♦7 that the ♦K is not going to bring down the ♦Q. North must be patient, lead something else, and wait for partner to get in and lead a hoped for third diamond. North does best to exit a club in case partner has the ace and declarer has the ♠Q and ♥J. As it happens, South doesn't have the ♣A, but has enough strength in clubs and spades so that declarer cannot come to a ninth trick without letting South in. Once South gets in and leads that remaining diamond, it's curtains for West. More often than not, third hand's return of the suit partner has led (♦7), sets the tone for the defense.