I haven't had one go off while wearing it, I would think with the rapid inflation that it would give you quite the wake up call if your not expecting it, maybe even if you are.
I've watched a couple video's of people testing them out when the HIT inflators are due replacement. I'll have to check the dates on mine, I kind of want to jump in the water to try it out when it's time to replace them. I think I'm good till the end of 2024.
I almost scared the out of Me. The inflation rate is instantaneous!