Unity trees impostors

Well I’m pretty sure I’ve seen them somewhere, but how to set them up?
I can’t find anything in the manual and also a google search does not return many results…

More, in this thread a user says that Unity’s trees impostors work quite well… I’m getting mad.

Oh cool, found another post about it.
Apparently, the impostor feature only activates if trees are painted on the terrain, but it’s not active if I just drag a Unity tree as a stand alone object.

Is this right?