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It seems the keyboard is pressure sensitive (reference to a pressure sensor in the HW section)

Pressure sensor is in the mouth piece.

I've always thought saxophone keys were on or off

Actually the pressure sensor is on the PCB. Pressure is transferred from the mouthpiece through a flexible silicon pipe. You can see the details in the assembly instructions here: https://github.com/jcard0na/haxo-hw/blob/main/docs/assembly....