HTC One M8 User Manual page 163

2. Tap to turn on Face Tracking. As other people around you enter
the front camera's field of vision, each person will appear in a
window on the video chat screen. Your camera will zoom in or out
to keep the faces in focus when you move.
3. Drag the mini-window anywhere you want.
4. Depending on the video chat app, you can single or double-tap the
mini-window to switch screens with the called participant.
5. Tap again to turn off Face Tracking.
Video chat and phone calls 163