It’s basically a potentiometer with a switch at the ‘10’ position on the knob. When the tone is set at 10 it clicks and the tone control is essentially removed from the circuit. This gives the pickup a wide open full treble sound. It replaces the 2nd tone knob and works the bridge and middle.Now on the newer models it works as a system in conjunction with the delta tone pickups(alnico 5)
RIPPED FROM THE NET:The 250k no-load tone pot (not included) functions as a normal tone control from 1-9, but at 10 takes the tone control out of the circuit leaving your guitar "plugged straight in" for more bass, high end and output. OEM on American Standard