how footswitch work ?

yes, as nik said it is true bypass, meaning that when the effect is off or bypassed the guitar signal goes straight from input to output.
The pin7 of the DUE will "feel" when the 3pdt footswitch is activated or not, the pin will be high when the effect is not working and low when is on.
You can have a look at this for more info:
www.electrosmash.com/forum/pedalshield-u...shooting?lang=en#535

I hope this image helps you:
- The green is the signal flow then the effect is not working (true bypass mode).
- The red flow is when the pedal is engaged.





The 3PDT works connecting the middle row pins with the top pins or bottom pins depending if is pushed or not, check this again:
www.electrosmash.com/forum/pedalshield-u...shooting?lang=en#535

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