First of all, you can't use this system without changing your eprom!
Air injectors work exactly opposite to the normal function of boost control.
If the injector is closed there's no pressure to the waste gate and the turbo is allowed to make as much boost as possible.
If you want to decrease the boost you open the air injector a bit and apply some boost to the waste gate, thereby opening it.
So, the higher the duty cycle on the air injector, the lower the boost is.
Normal boost control is the higher the duty cycle, the higher the boost is.

The advantage is that less air is blown off than with normal boost control.
For motorsports applications (with restrictors) this is very nice to have.
On an unrestricted turbo the gains are very small (if there's a gain).