A video going around the interwebs shows a snowboarder deploying an avalanche airbag as he realizes he’s caught at the beginning of an avalanche.
Do these airbags actually work? If so, how? I imagine it has to do with buoyancy, but I’m not quite seeing how the behavior of snow – to the extent it behaves like a solid rather than a fluid – could result in the airbag keeping the snowboarder “afloat.”
Not sure buoyancy is the right term, the airbag isn’t making them float. It’s based upon the idea that when a large amount of varied sized objects are shaken together the larger items rise to the top. An avalanche is basically that, a large amount of varied sized chunks of ice and snow being all jumbled up as it travels down a mountain. Fine snow and smaller pieces of ice will naturally end up on the bottom and larger pieces will be on top. A person with a deployed airbag is increasing their size and chance of staying on top of the avalanche.
Avalanches are scary and it is a horrible way to die. I was planning a backcountry heli-trip in British Columbia and did a lot of reading on the topic. Basically I psyched myself right out of doing it. Haha
Edited to Add: I found the term that applies, “Inverse segregation”. 
Data is sparse so far, but there is strong anecdotal evidence that it can help in some common types of avalanches that skiers encounter. It won’t do much good if you’re headed into trees or over significant drops, but it can help you from being buried in a flow.
But the best tool is to learn to avoid avy conditions. The guy in the video made some classic mistakes and should never have been there. It’s possible that wearing a pack like that helped him decide to ski where he shouldn’t have gone. It’s a real problem with safety equipment like this, it doesn’t necessarily result in fewer deaths or injuries.
Thanks for the handy search term. Searching on it revealed this web page, which discusses it in great detail. It seems that particle size matters, but density also matters: if the density ratio (between the large/small particles) is substantially greater than the diameter ratio, then the large particles will go down instead of up.
Interesting to note that it’s also been called “the Brazil nut effect,” due to Brazil nuts mysterious rising to the top of their smaller peers in cans of mixed nuts.
The general phenomenon is called granular convection, of which inverse segregation is a subset.
So an avalanche airbag improves both parameters for you: it makes you into a bigger particle (increases your diameter ratio with respect to the snow particles), and also decreases your overall density (decreasing your density ratio with respect to the snow particles). Even better, it puts the extra buoyancy near your head, so you’re more likely to come to rest in a head-up orientation. In the best case your head will be above the surface so you can get fresh air, but absent that it will at least make it easier for rescuers to quickly uncover your head.
Also to note that they bag will power deflate in 3 minutes (not just stop pumping air in, but reversing it ‘sucking’ air out), the idea is if you did get buried the 3 minutes would allow the snow to harden around you giving you a substantial air space for breathing without the snow collapsing in on it, perhaps even a space to the open air on top.
It also adds some protection to your head from trauma, which is a major factor in avalanche fatalities. But it’s still best to not get caught in an avy.
Fascinating stuff – the things you learn on the dope!