all theyre sayin is if you got a big hollow at the edge of your wall or your ceiling line a humongous great trowel will flatten it out for you....seein as they dont really get any bigger than about 20" the next best thing is a derby, like a bit of flat aluminium, maybe 4" wide and 4 foot long, two handles on it with a lip on one edge to keep it stiff/flat...
nothin to stop you using it to flatten the whole wall off if its a mile out, it can be done to a certain extent, but your only talkin 5mil on a coat, over 2 coats maybe 6-7mil, any thicker than that and youll have problems troweling up cos of where its on real thin versus where its on real thick...not to mention the ripples on the thick bits...then timing goes out the window and sheer experience takes over...