Yeah, sorry. I'm not great at explaining but I''ll try...
Rugby is a zero-sum game. If one team gets and advantage then, by definition, their opponents must be disadvantaged.
So, if team A knocks on and gets and advantage then I blow the whistle.
Also, if team A knocks on and does NOT gain an advantage then I call "Advantage Over" at some point.
I do this because the lack of advantage to team A must mean that there is an advantage to team B.
( Of course, one could argue that there has been no advantage to either A or B but I can;t think of an example where that would realistically happen )