In the Face Keys settings section, whenever you make a change a key will be added to the Face Track. This will cause the character's facial muscles to react accordingly.
Sprite-based heads have no muscles on the face, so how do they react to face keys?
A built-in face feature template is actually a sprite that contains one or more elements. For example, one of the eyes contains multiple images that represent the different expressions within the eye.
When you select the solo feature (eyelid for example) in the Facial tab and drag downwards,
or apply any one template from the Template tab: