On many uprights, the middle pedal is referred to as the “practice pedal.” This pedal lowers the mute rail felt between the hammers and the strings which significantly reduces the volume of the piano. A properly regulated practice pedal positions the mute rail above the hammers so that they clear the bottom of the felt, but low enough when activated so that all hammers are muted consistently from end to end of the piano.
Properly Regulated Mute Rail:
On many pianos, this adjustment is made at the pedal nut:
On certain common Yamaha pianos, the adjustment is made by turning the white thumb screw on the vertical rod above the middle pedal:
Twist the thumbscrew clockwise to lower the rail:
Twist the thumbscrew counter-clockwise to raise the rail: