From 3Leaf:

The Chromatron is an analog state variable filter like no other. It can switch seamlessly between envelope control and manual control via expression pedal, unlocking psychedelic tones and textures that will surprise and inspire. It features an original envelope detector circuit that follows your instrument dynamics in a musical way, and a low-noise signal path using only audiophile-grade components.

Press the MODE footswitch and the filter cutoff is controlled manually via expression pedal. The expression pedal sweep has been fine-tuned to be smooth and progressive, allowing you to achieve lush, synth-like filter movement.