What the heck is a Fender "Bright switch?"

The "bright" switch on every vintage Fender amp is a switch that puts a small value capacitor (100 picofarads in tweed amps, 120 picofarads in the blackfaces, and 47 in most Deluxes) across the volume control, from input to output. Since higher frequencies see a small capacitor as an almost dead short, the high frequency signal bypasses the volume control altogether. The lows and mids get attenuated by the volume control, but the very highs go around the volume control through the capacitor without much attenuation. We hear this as bright. For this reason, the control is more effective as the volume control is turned down and conversely, the "bright" switch will have hardly any effect when the volume is turned up. If the volume is all the way up, it will have no effect whatsoever.