How to double Line6 Stompboxes presets? The solution is here, thanks to that mini switcher which allows to simulate the High and Low positions of an expression pedal.

Just connect it to the input "expression pedal" of a DL4, MM4, DL4... It has a bicolor led indicator to know the current position.

The external power supply +9Vdc, BOSS format, is useful for the led indicator. But, the switcher works well without that supply (in this case, the led will be off of course).

Finally, to connect it to a Line6 Stompboxe, you have to use a stereo jack.