Somewhat magical.
All bound expressions in Vue are javascript expressions. Names are typically bound on "this", which is the component instance. So the above expression is equivalent to
this.toggleVar = !this.toggleVar.Not entirely true -- I know there's at least special processing for filters: https://vuejs.org/v2/guide/syntax.html#Filters
Oh boy. Another reason why I don't like templates -- you invariably end up with magical incantations like this. Working with JSX has given me an incredible distaste for templating languages.