The signal leaves the phono stage and enters the Stereo-Mono switch's PCB. Then you have a choice between stereo, which just passes the output signals out; or mute, which floats the phono stage outputs and grounds the switch's outs to ground; or mono, which combines the phono stage's output signals via two resistors. Since the rotary switch is a shorting type (a make before break type), there are no popping sounds between clicks.
Kit includes the PCB, rotary switch, and several pairs resistor values to choose from, either carbon-film or metal-film types. Amazingly inexpensive and amazingly effective.