The key new feature can automatically detect if you've had an accident on your route. Incident detection can also be activated during an outdoor walk, run, hike or cycle where your Garmin device will automatically send your emergency contact a message. The message will include your current location. It'll be automatically turned on when you add an emergency contact to Garmin Connect and it gets synchronised to the watch.
You can also manually choose to send a message to your chosen contact using an "assistance feature" should you get into difficulties. On the 645 Music and Fenix 5 Plus you will need to activate this by holding down the top left button for approximately seven seconds until the watch vibrates three times.
You can also control this via Settings > Safety > Incident detection. On the Vivoactive 3 Music, you'll need to go to Settings > Safety & Tracking > Incident Detection.
There is a bit of a drawback in that the features do require your smartphone to be present and be running Garmin Connect - so it's not a great deal of use if you regularly exercise without your handset.
The features will come to the Forerunner 645 as well soon.