This is kind of interesting:
Here’s a more concrete example: Imagine that your annoying work “friend” comes in one day carrying a brand new, very expensive handbag that you absolutely hate.
You could say: Wow, that bag is not cute. Why did you even buy it? But that would probably be incredibly awkward and lead to a fight. That is not shade; that’s outright hostility.
Instead, if you wanted to insult her bag by using shade, you could smirk: You’re always so adventurous when it comes to fashion. I could never take the risks that you do. I admire that. (The implication, obviously, is that her bag is fugtrocious and you wouldn’t be caught dead with it.)
In many ways it seems a lot like negging.