I crashed the other, incident detection did not work

It's definitely not something to rely on.  It's just an adjunct that might help in some cases.  Of course you need to have your phone and it needs to have a cell signal for it to notify anybody.  I mainly ride mtb and often in areas with poor or no cell coverage.  Also, riding mtb I have somewhat regular mishaps.  Some minor ones trigger incident detection and some more significant ones don't.  It depends on the details of the accelerations experienced by the 840 and that can vary a lot depending on the details of the mishap.

Last week I crashed and the feature worked well.. I think it depends on how you crash: if the 840 stops moving all of a sudden or if it has a kind of shock in the movement it recognizes the crash. If you "just" slide down maybe it doesn't feel the crash.. 

I guess you didn't crash hard enough :)

it's just based on gyros and/or accelerometers.

I always have my phone with me as well. So in worst case I could have called myself. I guess I wasn't necessarily relying on the feature since I wasn't planning to crash in the first place :) was more an afterthought of "why didn't that thing notify my wife". 

well after analyzing the GPS data I was rolling ~46km/h, I had applied brakes so maybe dropped a bit, but crash definitely felt hard to me ... not to be dramatic but I ended up with minor fractures to the traverse process L2, L3, L4, and a dislocated finger. Which was worrying to me when the doctor said it, but no surgery required and I can even get up and walk. Just rest for 4-8 weeks and I should make a full recovery :D So in the end I feel ok about the incident and looking forward to getting back on the bike ASAP.

sorry to hear that :/  mine activated 'incident' many times, even when there was no crash, maybe just like throwing the bike into a ground (jumping from it).

I hope you will heal fast.

thanks - ideally I won't need to test the feature again :)