Eye tracking is something of a hot topic in the Google Glass community, because it enables the user to perform tasks without voice or touch. The first thing to ever use the eye sensor on the inside hinge of this wearable computer was a third party app that allowed users to take photos with just a blink, which did not help the conversation at the time regarding whether or not Glass could be used to violate the privacy of others. Google has since enabled winking to take a photo as a beta feature that can be activated at the users discretion, but now there's a feature that uses the eye sensor to remove some of the awkwardness associated with using Glass.
Glance Notifications make it possible for you to wake the display and check on an incoming notification just my looking up at the Glass display. When Google Glass has an incoming notification, the user hears a chime through the bone conduction speaker near the ear. Until recently, your options for checking this notification were to either tilt your head up to the preset wake angle or just tapping the touch panel on the side of the device. Since the whole point of Glass is to operate hands free in as many situations as possible, the head tilt quickly became the most used method of checking notifications. Unfortunately, since no one but you can hear the notification chime, this meant you looked a little awkward when checking notification in public. In fact, this is one of the big points that was used to poke fun at Glass during the SNL skit that happened just after Glass was first released to Explorers.
Like all things that involve the eye tracking sensor, there's a quick training walkthrough you have to use to calibrate the sensor to your eye. It's quick and painless, though I have found that it seems to calibrate more accurately for all environments if you go through the training in a darker room. It's basically measuring your eye movement, so when Glass detects the right amount of movement it can wake up the display. Once you've calibrated the sensor to your eye, the next time you get a notification tone all you need to do is look up with your eye. By the time your eye has reached the display, Glass will be ready to show you the notification. Like any other Glass notification you can choose to either ignore it or interact with voice or touch, but the important thing is that you've seen the notification without any awkward head gesture and still managed to be hands free.
This is the kind of thing Google Glass needs in order to be relevant in a world of incredibly nice looking smartwatches. The whole point of Glass should be to act as a digital helper in environments where your hands are busy. Voice commands and eye tracking are a big part of that equation, and it's great to see Google moving in this direction.