I'm not positive, but I think you're misunderstanding what the manual is describing... Forgive me if I'm misunderstanding!
The HX can, with certain amps, act as a foot switch to trigger functions on the amp. It's much more common on guitar amps, which frequently have switchable reverb, tremolo, drive channels, etc. It's not to act as a secondary output from the HX.
In other words, it's not carrying an audio signal. It would just be sending a signal to tell the amp to switch something on/off. So I think @Tandark is talking about using that to trigger preset changes on a Strymon pedal. And to be clear, I have no idea if the HX can act that way (at least not without MIDI).