Max defroster raises the temperature and fan speed. Air conditioning is activated and air recirculation is unavailable while max defroster is active. When max defroster is turned off again, the climate settings return to their previous levels.
Note
When max defroster uses the high fan speed, the noise level of the fans increases.
Press the fan symbol 
in the bottom bar and go to Climate.
in the bottom bar and go to Climate.Press Max defroster
.
.Tip
The defroster button 
in the overhead console activates max defroster as well as the heating of the rear windscreen.