With the fingerprint scanner in the back this seems like a moot point. The fingerprint scanner does an awesome job of waking and unlocking your device. The Nexus 6 didn't have a fingerprint scanner so a better way of waking this device was to double tap the screen. The feature does exist in the Nexus 6P, but it is hidden. In order to enable the feature you will need to be rooted. Once rooted you will need to copy the code below into terminal emulator.
echo 1 > /sys/devices/soc.0/f9924000.i2c/i2c-2/2-0070/input/input0/wake_gesture
The only real downside to this is that it will not stick through a reboot. Which means everytime you reboot you will have to retype the code which is kind of lame!
via XDA