This is only available for Lollipop users, so I haven't had a chance to test it personally yet. It uses built in sensors like accelerometers to determine whether someone is carrying or holding the device. When it senses that movement has stopped, it will lock the device and revert to a lock screen and require a PIN or pattern to unlock. The idea is to make it more convenient so that people will actually use a PIN (or other method) to secure their devices. I'm always amazed at how many of my friends don't bother to lock their phones, despite the risks.