The acceleration your car experiences in an accident is an order of magnitude higher than anything your car ever experiences in everyday use, making the control system very easy to engineer - something like anything over 20g's and it pops (the algorithms take vehicle speed into account too for a varying threshold and probably use jerk.)

In that video where the rider was hit from behind, there was very low relative change in velocity of the sensors in that helmet - no direct impact spike - and its on par with the same levels of acceleration you would experience in everyday bike riding.