It's used like in the iPhone. Press the green button to answer (note that the screenshot above is in the middle of an already answered call, that's why it's showind End), when talking the proximity sensor detects your face next to it and turns off the screen to save energy.

Not sure if you get a call and the phone is in your pocket tho.
I suppose it'll answer (as regular phones do with regular keys) if you accidentally press on the button...