Hi,
Face detection is becoming more important as an selective tool. Face Detection can also be an interesting motion algorithm to reduce false motion triggers from snow/rain, moving shadows etc. However, a warning that the algorithm is sensitive for a face and if you have a moving target in general it will not trigger the motion detection system and you will miss important information. Therefore, I suggest you test how it works in your situation.
For the camera go to the settings for Motion Detection.
In the previous menu select View → Motion
Now, view the video from the camera. Direct the camera to yourself and when your face show up in the camera view the Face Detection algorithm will indicate that it has detected a face with a red frame. The motion detection system is triggered and a recording starts.
I can wave around with a lot of stuff in front of the camera with no trigger. However, it will most likely be sensitive to how much light there is in the field of view, distance between camera and object, camera resolution etc. This is a basic algorithm for a human face. If it will work for your dog, cat, cow, robot or whatever you want to detect you have to test. So it must be tested and verified that it works correct in your setup.
Good luck,
Henrik