IIUC, "@--+::" should read "all descendants of the children of the grandparents of the current commit".

Or if it were a family tree, "all descendants of your parents and their cousins" (ignoring that family trees are way more constrained than DAGs).

I have no idea why you would use that revset; I assumed it was just an example of revset's complexity and power.

As a pragmatic matter, unless I'm scripting with revsets, I personally stick to a few go-to aliases.