HTC One M9 - Manual (Page 30)

Get Started 17
Three-finger Tap
While using the HTC Car app, tap the screen with three fingers to activate the voice command
mode.
Motion Gestures
Use motion gestures to rotate the screen, mute, lower the ringtone volume, and more.
Flip to Mute
When a call comes in, you can turn your phone over to mute it.
To change the Flip to mute settings:
1. Tap > > Settings.
2. Tap Sound & notification >Flip to mute, and select a setting: