It's less of putting a blanket on and more of filling the headspace. When your mead naturally puts off CO2 during your ferment, it fills the space and pushes out the airlock. If you keep your setup closed, most of whats in your carboy/bucket will remain CO2. If you open it near the end of fermentation and introduce some normal air when its still degassing, and then open it up later, the effect is more noticeable. If in this example, I had 4 gallons in a 6 gallon carboy, I'll notice that the top gallon is more like normal air, but if I put my head in the bucket below the 5gallon mark, I can notice the concentrated CO2 sitting on the must, because it will make your nostrils burn.
Once you open it at this stage though, you're pretty much dispersing whatever is left. To put CO2 back in as a protective layer, you can't just spurt a little in. The air currents from doing that alone won't let it settle. You have to purge it, by filling up all/most of the headspace with CO2, with the lid on to trap as much as you can. I know a lot of beer brewers who purge their kegs like this before kegging, although it seems like a waste of CO2 since you'll be purging with CO2 after you transfer, but they are nuts about oxidation.
And Bernard, the CO2 in the atmosphere is like 0.04%. Even if it did settle, you wouldn't notice it I think. For our purposes in the bucket, we would be looking at like 50% or more.
Squatchy also mentioned in another thread that topping off with CO2 isn't an effective long term protective measure, especially compared to sulfite additions. I know wineries use inert gasses like Argon, I imagine because its inert and won't react with anything over time, but CO2 might. But CO2 is cheap and argon is not, and its certainly helpful even if only by a little.