Gatty should visit Mike Easton's web site ,
https://customchromatics.com , for pictures and complete information on the HFC and its accessories. There is also a diagram and a description of how it works.
Foot-Pedal (or your string tied to your toe) systems use the standard slide of the chromatic and do not require any substantial modification of the instrument. However, the foot pedal anchors the performer to one spot on the stage and is cumbersome to transport.
The HFC mechanism replaces the mouthpiece and slide of a standard chromatic. It allows free movement of the performer about the stage and adds no cumbersome claptrap to be protected and transported.
I experimented with and used both systems for many years.
Vern