Came across these earlier. Actually it's not only triggered by impact. There's a gyroscope that detects if you are leaning over too far, and that triggers the airbag as well.
Unfortunately that means you can't ever just stop and lean over to tie the shoelace that just worked itself loose, because that'll set it off. You also need to remember to turn the thing off before you actually take it off.
It's a one-time mechanism, this. Once it is triggered, the whole thing needs to either go back to the manufacturer or be replaced.
My main issue with this is that it's just a collar really. If you want to compare this to a normal helmet, compare it to putting on a helmet and not using the chin strap. It can easily be pulled off your head on impact.