Notification Glance is an alternative to Head Wake that lets you respond to Glass with just a look.
Notification Glance is not on by default, but you can enable it by going to the Settings bundle. When you turn it on, it will have you calibrate the feature. Afterwards, you can test and recalibrate it if Glass responds too often or too little, or turn it off.
When you hear a chime from your Glass, it means that something new has appeared that could use your attention. Maybe one of your friends sent you a message, or you have an appointment coming up. With Notification Glance on, you can check by looking in the direction of the Glass display. This will wake Glass, and display the notification that just arrived.
Turning on Notification Glance will disable Head Wake.