The feature works by instantly turning your Android device into a hotspot for your Chromebook, assuming they were both using the same Google account. This was a quick way to get online with your Chromebook even without a WiFi connection.
Up until now, Chrome OS has had bits and pieces of support for instant tethering that could be manually turned on but didn’t always work well, but according to the Chromium Gerrit, it’ll be shipping with the feature turned on by default soon. No tinkering on the user’s end, just make sure everything is signed in and you’re ready to go.
Users on early and dev builds of Chrome will see this before everyone, but if everything works as it should, we can probably expect it to roll out into the mainstream channels in the next few months.
source: XDA Developers