The new Apple Watch Series 4 has sensors that can automatically detect if the wearer trips and falls. It's ideal for people who might be prone to falling, such as the elderly. If someone with the new Apple Watch falls, and has the new feature activated, a pop-up will ask if they're OK. If they don't respond, it will sound an alarm and then automatically call emergency services.
The feature is off by default, since Apple says "high impact activity" can appear to be a fall for some people. If you want to turn it on, here's how.
When I tested this feature during our review, I noticed it sometimes takes a few seconds laying on the ground before it activates, but it seemed to work most times when I fell down purposefully in a gym. It's not 100-percent fail-safe, and Apple warns that it can't detect all falls.