Turning ON/OFF Tracking
Press AF Point Main button (top right back of camera, right of astrek):
Locking the tracking to Subject
After you exit the AF Point Main button, then you can press Multi-Function-2 button to lock the tracking on the subject.
How to focus on one of six faces
If you have six faces in the same frame and only want to focus on one face then only make the Flexible Zone AF1 box the size of one face. You can use that box with subject tracking ON to lock tracking on that one face and recomposed.