that's because...if sensor is covered... if the sensor is covered ..it will think that the phone is in a pocket..so always display off automatically to save power.. light up and down according to environment.. if you're in a bright area ..like light bulbs ...or Sun light time...then the always on display will light up..so you can see it..if you go into dark places or dark room.. it will auto.atically go down..due to over brightness..
