The Master Filter was designed as a basic tone control. While this easy-to-use filter has a very good sound and requires quite a bit less CPU power than the Power Filter, it cannot be modulated like the Power Filter.
This fader activates a Low Pass Filter when moved left-of-center and activates a High Pass Filter when moved right-of-center. Only one filter is active at a time.
When the fader sits at the center default position, the Master Filter is disengaged. Once the slider is moved, it is engaged and will remain engaged until you Right/Control-Click the slider.
This fader controls the Emphasis (also known as Resonance or Q) of the Master Filter. Moving this fader to the right increases Q (narrowing the bandwidth and emphasizing the filter frequency). Lowering this fader decreases Q (widening the bandwidth).