The Lollipop Smart Lock functionality is pretty cool. You can have the phone stay unlocked when it's connected to a trusted Bluetooth device, in range of a certain NFC tag, or when it sees a trusted face (presumably yours). Trusted Face mode in particular is quite cool, but it's not necessarily as secure as a PIN or pattern lock. You can, however, temporarily switch to the secure lock screen with a single tap.
There's a little icon down at the bottom of the lock screen that tells you the unlock status—either looking for a face, locked, or unlocked. To lock the device when the secure lock screen is disabled by Smart Lock, just long-press that icon. If you do this, you will have to manually unlock the phone once, but after that it switches back to Smart Lock (whichever mode you have enabled).
This is handy for times when you might be in the room, but don't want your phone sitting around completely unlocked simply because your trusted device is in range. It's the same for Trusted Face, but that doesn't seem as useful since you'd have to be holding the phone for it to work. Yes, someone could use a picture of you, but who's going to go to all that trouble?
[Thanks, +David Schmidt]