The behavior of the tilde modifier is interesting. It appears to respect other modifiers like "-" (the "not" modifier). It does not appear to respect meta-tags.

It appears that e621 considers ~ to be more like a "at least one of these" modifier - it will return posts that have at least one of the tags modified by it.

• So ~cat will return pictures with the cat tag, the same behavior as cat with no modifiers.

• ~cat ~dog will return pictures with either the cat or dog tags.

• female ~cat ~dog will return pictures with the female tag, which also contain either the cat or dog tags.

If you wanted to search for your favorites which contain some "x" or "y", you might try fav:JPone123 ~x ~y. Or, for more concrete example, maybe fav:JPone123 ~fluttershy_(mlp) ~applejack_(mlp).

It would be pretty cool if there were better tag-grouping or boolean support in e621, but at the same time I don't feel like it's really "missing" for any of my purposes, so I'm not going to be the one to make a feature request here. :3