Google appears to be quietly rolling out a new addition to the Smart Lock feature in Android Lollipop, bringing an extra unlock option that works with motion sensors inside Android devices. "On-body detection" is designed to use your phone's accelerometer to figure out when it's been set down on a surface, and lock — with whatever lock pattern or PIN code you've set — when it's not on your person.
After unlocking, if the phone detects continuous movement, indicating it's in your hand or pocket, it'll stay unlocked until the next time it's motionless. We've tested on-body detection on the Nexus 6 and it works pretty much as advertised, though it doesn't quite lock immediately after you set your phone down — it seems to have a timeout of a few seconds.
Keep your phone in your hand or pocket and it'll stay unlocked. Set it down and you'll need to enter your PIN or pattern.
Whether you should actually use this kind of feature is another question entirely. It's potentially much less secure than tying your lock screen security to a Bluetooth device on your person, or simply keeping a passcode on at all times. It is perhaps preferable to no security it all, though, and Google's probably targeting those who wouldn't normally bother with lock screen security with on-body detection. Nevertheless, it's far from an iron-clad defense for your personal data.
We've spotted on-body detection on some, but not all of our Lollipop devices — phones running both Android 5.0 and the newer 5.1. In traditional Google style, it seems to be slowly trickling out across the ecosystem, so don't worry if you're not seeing it yet. Head to Settings > Security > Smart Lock to see if it's there, and be sure to hit the comments and tell us how you're getting on.