In the words of GFI System:

Rossie is a dynamic multi-filter pedal, encompassing 3 essential tone-enriching filter effects:

• Envelope Filter – a colourful resonant filter that follows your picking dynamics.
• Step Filter – a groovy, arpeggiated filter with fully adjustable steps.
• Manual Filter – great for static tone shaping or sweep-able wah-like effect using an expression pedal.
• Each filter mode offers 4 filter types: lowpass, bandpass, highpass, and peaking. Sweep range and resonance width are also adjustable. And on top of all of these, a dedicated Mix knob allows blending of wet and dry signal, this gives you total control of how much filtering effect is imparted on your guitar or bass signal.