I think the impact it detects has to be pretty significant. Like, you’ve got driving focus right? It detects the iPhone moving because the car is moving. Which is why often times driving focus will activate for passengers. It doesn’t distinguish between driver & passenger.

If it detects the iPhone in motion, then suddenly — BAM — the phone is stationary, it could trigger the detection.

They demonstrated this in a granted, factionalized context on a recent episode of one of the Law & Order iterations. Chris Meloni’s character was in a tight spot, borrowed the phone from a fellow detective, and chucked it as far as he could outside of a dead zone. Phone detected a sudden cessation of movement and automatically dialed 911.

Again, that’s obviously fictionalized, but they got the idea from the real world tech.