The Nexus 6 Double tap to wake was originally intended to have this feature enabled by default but for some reason it was disabled before Google rolled out Android Lollipop 5.0. This does mean that the feature is still on the device and just has to be enabled, XDA user Naxir managed to do just that. (Keep in mind that this requires a rooted Nexus 6, you can learn how to root it here)
Now if you double tap your Nexus 6 display while it is locked, the device will automatically wake up. This could lead to some battery issues as users do accidentally tap their device with frequency.