Shake for Emergency is a feature that allows you to activate the Emergency feature, even when your phone is locked.
To trigger Shake for Emergency, you need to quickly rotate your phone back and forth around the longest axis of the screen for three seconds:
When you have successfully activated Emergency using the shake gesture, you will receive a notification (for instance a lock screen notification) to let you know that you are now in Emergency mode.
Caution: Because Shake for Emergency requires your phone to stay active in the background (for instance when your phone is locked) and listening to your phone's location services, then Shake for Emergency is one of the features that consumes a noticeable amount of battery power.
You can enable or disable this feature from the Settings: