Nexus users will soon be able to use a "Trusted Voice" to unlock their stock Android device
The feature uses the same process used to train your phone to accept the "O.K. Google" command. Google warns you that this is not as secure as using a PIN or a password because someone with a recording of your voice can fool the lock as could someone with a similar sounding voice. You are asked to come up with a PIN just in case you are ill and you don't sound like yourself.
Besides Trusted Devices and Trusted Faces, the Smart Lock feature also includes Trusted Places which unlocks your phone when you are at a certain location. On-body detection keeps the phone unlocked in your pocket, but locks it on a desk or table. Soon, Trusted Voice will be added to the equation. If you own a Nexus device, start prepping your voice, because this will soon be available for your stock Android device.
source: ArsTechnica via SlashGear