Ah I always thought dust off was just CO2 compressed in a can, now that I am looking at a can of the stuff (dust destroyer from staples) it does state that there is difluoroethane in it. Doesn't say the percentage though, and it also contains a bitterant too. I personally dont use these products to fill up the empty space in a bottle, I usually optimize my developing routine to use up a whole bottle of stock solution (I separate them out to 1L bottles) or if I cant use it all up I squeeze the air out (burping the bottle) before I seal it. Since I only use plastic bottles this is possible, but can see why this product would be helpful if using glass bottles. I have also read of people using clean glass marbles to displace the air space, or separating liquid contents from the air with floating lids in larger tanks.

This finding could be useful though no? If someone would want to boost the activity of the developer without increasing the temperature, this could be a way for extra push processing without having to worry about the emulsion in a hotter/warmer developer. You could stick the end of the dust off can into the developer and bubble it in, or maybe fill half a bottle with it and the developer and shake it in? Maybe there is too much variation in the off the shelf products or even in the absorption rates for this to work well though.