HTC One M8 for Windows - Manual (Page 34)

Motion gestures
Use motion gestures to mute the ringer and more.
Flip to silence
When a call comes in you can turn your
phone over to mute it.
Pick up to lower volume
In a business meeting or restaurant,
pick your phone up when a call
comes in to lower the ringtone
volume.
34 Your first week with your new phone