The Z Air Brake has two major applications. One is for stage use, the other is for studio or home use.
For stage use, the Z Air Brake is a useful tool to limit your overall dB level at any time during a performance. Example: the beginning of the night (when the club isn't full) click the Air Brake to a level of attenuation to allow for smooth tone at a lower volume level. As the night progresses, reach back and remove degrees of attenuation until the desired level is reached. This will allow you to manage your amp's overall volume without varying from your amp's favorite settings.
At home, the Air Brake is useful for lower volume while still allowing for full output tube distortion. The Air Brake has a "bedroom" level control that, when selected, will allow for fine adjustment and maximum attenuation down to 1 watt (with amps less than 100 watts.)