When your phone is closed, or folded like a tent or stand, you can silence the ringer when you bring your hand close to the external display and then pull it away.
You can turn this gesture on or off for:
To turn it on or off:
If you want the gesture, turn Use Approach to silence on 
To choose what to silence, tap 



You can also set the approach gesture to show your external display lock (or home) screen when you’re in desk mode.