Using Smart Lock on Android 5.0 is a great way to balance security and convenience, using a trusted location, Bluetooth device or NFC tag to bypass lock screen security without entering a PIN or lock pattern. However it's possible you might not want your phone completely unlocked all the time, which is why there's a simple way to manually lock your device — requiring your PIN, pattern or password to unlock — with just a button press from the lock screen.
Once that's done, head to the lock screen while you're in a trusted area, or paired to a trusted Bluetooth device. You'll notice a lock icon at the bottom of the screen, in the middle. If you're not using Smart Lock, tapping this doesn't do anything. But if you are, this is your manual lock button, and pressing it will do just that. That means to get past the lock screen you'll need to enter your security pattern or PIN, just as if your device wasn't in a "trusted" state.
Keep prying eyes away from your personal stuff — at least until you next unlock your phone
Once you unlock, everything goes back to normal — subsequent unlocks won't require additional security, assuming you're in a trusted place or paired to a trusted device.
We're seeing this feature on "stock" Lollipop devices like Nexus and Google Play editions, as well as the Moto X when running Lollipop. As always, manufacturers are free to tweak the lock screen to their own spec, so customized Android 5.0 phones from other manufacturers may be missing this feature. But if you do have it, it's a neat way to keep prying eyes away from your personal stuff — at least until you return to your device to unlock it manually.