I'm having difficulty internalizing the $\diamondsuit$-principle. I understand what it says, and can see why some of the constructions that I've seen that use it work. But I don't think I understand it well enough to know when/how to use it more generally.
Is there any "intuitive" motivating idea behind $\diamondsuit$? Or is it just an arcane principle that I'll need to learn to work with on its own terms?