Facial classification and tracking is easier compared to tracking any other motion. This is due to the fact that the face have a large number of easily identifiable features.
Facial tracking is an additional layer on top of facial detection. Facial detection works by finding characteristics such as the cheekbones, chin, nose, eyes etc. These features are easy to detect as they have very specific properties. These points are found with the help of shadows and brightness, for example, the nose and cheekbones are highlighted, whereas the the eyes and lips have a shadows.
Using these feature points it is possible to create a mesh to construct the face. This has been a classical way to detect and classify faces. The TrueDepth camera helps to make a better structure by providing depth with the help of an IR sensor.
Facial tracking makes use of this mesh to study and track changes in the facial structure. For example when you smile, it will detect the cheekbone points changing, similarly when you open your mouth, your lips will move apart.