@DeathDr. I dont want to pin point but there is a small mistake in your definition for Expression Pedal.
Expression pedals are not input devices. They are basically Potentiometers or Variable resistors. Let me give various examples of how it us used.
Pre-Distortion Volume Pedal: Offer resistance to the input guitar signal. With more resistance there's more voltage drop across the resistance which results in a cleaner tone even with crunchier distortion. At the lowest resistance your volume is full with very minimal voltage drop across this resistance.
Expression Pedal Assigned to Gain: This is again variable resistance which controls the amplifiers gain. Higher resistance causes lower gain.
Drive: This is a variable resistance connected between the two stages of the amp. The first stage is usually a buffer offering high input impedance so that it doesn't cause loading effects on your guitar. There is a slight signal strengthening also at this stage. The output of this signal is given through a variable resistance to the amp that distorts the signal. Stronger the resistance weaker is the signal strength resulting in softer distortion. It reverse for a strong crunchier overdrive.
Wah: Wah is basically a bandpass filter. The variable resistance changes the RC characteristics which affects the frequency of the signal.
I can talk about all the pedals but dont wanna bore with too much info.
Even in processors Expression pedals only change the resistance values of a particular circuitry. There are various buttons or switches which connects this variable resistance to the appropriate circuitry that giving you a parameter to change.
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