AngularJs: intersting syntax for complex ng-class conditions

Just now I got known quite interesting condition syntax for ng-class (from stackoverflow answer):

ng-class="{admin:'enabled', moderator:'disabled', '':'hidden'}[user.role]"

it seems that there is not good explanation for such case in documentation.
being inspired by this possibility I decided to create even more complex condition:

ng-class="connections.length && ({true:'open', false:'close'}[!!manager.showConnections])"

(flag open/close will be shown only if there are some connections for current manager)