Google has given users plenty of options when it comes to how they unlock their phones, and with some of the most recent versions of the software we've seen things like Face Unlock, and the more recent trusted devices and the new trusted voice which has been appearing as an option for some users who have the most up to date version of the Google Play Services apk. The other prerequisite besides having the latest Google Play Services version is that your device has to be running on Lollipop, and even then it doesn't seem to be appearing for all users with these criteria met.
If you are one of the users that have it as an option, you can choose to select it as opposed to something like a PIN or pattern lock, and from then on you could speak a voice command to unlock your device as the Trusted Voice smart lock features seeks out to learn your voice. Google has pointed out that this is less secure than other lock options though, and it shouldn't be used as a complete replacement if you want something with better security. Even if it is a less secure option, it's still kind of cool and many users would at least want to to give it a try whether they stick with it or not.
Anyone that uses a Nexus 6 however doesn't appear to have that option as Google mentions on the support page for setting up devices with automatic unlock, that the Nexus 6 doesn't support the feature. More specifically, it says "this feature isn't available for Nexus 6." Why Google chose to withdraw the Nexus 6 as a compatible device for the new trusted voice smart lock option is anybody's guess, but whatever the reasons may be the results are still the same, as it doesn't change the fact that Nexus 6 owners aren't able to utilize the functionality. At least that how it appears. Droid Life points out that users can still use the voice setting from the lock screen if they want to search for stuff, but just like with opening the camera from the lockscreen on lollipop and snapping a picture, backing out of the camera, or in this case tapping on a search result, requires the unlocking to be done via whatever method you have set in place, I.E. PIN, password, etc.