That’s the point. Partner doesn’t know what to return and is guessing between spades and clubs. Here’s where the suit preference signal comes into play. If you want the higher-ranking of the other two suits led back, you lead back ♦ 10; if you want the lower-ranking suit returned, you lead back ♦ 2. In (i), you lead back ♦ 10; in (ii), ♦ 2. In either case, partner leads back to your ♠ A/♣ A and you give partner a second diamond ruff.

It can get quite subtle, especially in expert circles. If you’re not sure what you want partner to return, lead back ♦ 5, the middle card. If you think you want a spade but you’re not sure, lead back ♦ 8; if you think you want a club but are not sure, lead back ♦ 4