Great little feature I use when running to keep my phone unlocked for easy access to my MyTracks stats etc.
Go to the Settings icon > Security > Smart Lock > [confirm your PIN if required] > Trusted Devices > + Add a trusted device > choose Bluetooth > and select your paired Bluetooth device.
I pair my phone with my heart rate monitor, this way when I clip it on to it's chest strap it's active and my phone unlocks. Once I finish my run I un-clip and the device is off and my phone returns to it's normal security preferences.
Supported OS: Android 5.0+
Set up complexity: Easy
Requirements: BT paired device you can run with