Once the processor locates and centers the camera's field of view on the user's face the processor may continuously adjust the position of the lid in order to maintain that centering.
For example, if the user is initially sitting down and the user then stands up, the processor will detect the change in the location of the user's face and adjust the position of lid assembly 12 by rotating toward the fully open position in order to have the user's face remain in the middle portion of the camera.
Full details at Patently Apple. The site writes the open and unlock procedures may have different levels of security. For instance, it could require authentication before unlocking the computer. This patent was originally filled in Q4 2013 and got granted earlier this month.