What is Crash Detection on iPhone / Apple Watch?

What does Apple Crash Detection do? How does Crash Detection work on iPhone?

iPhone Crash Detection

Crash Detection can detect a severe car crash and automatically connect your device to emergency services. The feature uses sensors to detect front-impact, rear-end, rollover, and other types of abrupt collisions when traveling in passenger vehicles.

The iPhone or Apple Watch sounds an alarm and shows an alert when a severe crash is detected. Your device will automatically start Emergency SOS if not canceled. After a 20-second delay, Emergency SOS automatically calls emergency services and sends messages with your location to your emergency contacts.