It seems that alongside its constant background recording of the last 5 minutes of gameplay, the Xbox One's DVR will automatically record gameplay every time an achievement is unlocked. The information was revealed by Xbox Australia product manager Adam Pollington in an interview with YouTuber ChampChong at EB Games Expo.
Pollington also said that the Kinect could be used to record the player's reaction whenever an achievement is unlocked, if developer decided to include the feature.
In the video Pollington also discusses the Xbox One Upload Studio, as well as the improvements brought to the table for Kinect 2.0. This includes the fact that Kinect 2.0 can now support 6 players, and that it requires less space (around a metre) to capture the full body. So if you have a small room, Kinect should now function a lot more effectively.
Thanks, Gear Nuke.