I don't know how this is done, can someone please explain how you do this. When you getting ready to belay someone on top rop using an ATC belay device I think some people wrap the rope around their carabeaner and as they belay someone up they keep pulling the rope by the carabeaner to keep the winding tight acting as a resistant in case they let go the rope.

I know some people do a knot on the rope below their atc device if they are belaying on a cliff so that the person stops where they are and doesn't fall further down the cliff if they let go the rope by accident.

How do you back up an ATC device withoug using another piece of rope?

If you mean to back up the ATC so that it only feeds in one direction, just put a prussik on the line and feed rope through ATC or prussik.

If you want a minimalist approach with only a crabiner just use a munter hitch, it works really well. I met some Germans who use it to belay sport lead, I wouldn't go that far but for belaying from the top its fine.

If you want to block off the ATC completely, lets say in the case of a fallen climber and you need to get the load off or do some work, you feed the rope through the carabiner and itself. A bit hard to explain I think its called a munter mule knot, if you do a search for knots on the web you will find it.

Thanks Gonzo for the information.