Understanding Amplifier Function
R75 senses the speaker current and the bit in yellow essentially provides a variable (R2) amount of positive feedback from the load current to raise the output voltage with the load current, by injecting current into the inverting input of U3A. 

Essentially this can be thought of as giving the amplifier a negative output impedance (Output voltage rises with load current), intended to cancel the drivers winding resistance and thereby raise the damping factor of the system.

It is almost but not exactly a current drive arrangement.

I would note that turning the damping control up too far could potentially compromise amplifier loop stability.

SW1 disables this loop by dropping the gain of U3B to zero leaving the thing setup as a conventional voltage amplifier with a little additional output resistance.