The "Y" function sends the input signal to both the A and B outputs in parallel. Always connected "thru" jack for use with a tuner or other piece of gear.
I use a Marshall stereo preamp. With this switcher I can add a center channel. The in and out of phase switch is terrific when used with 2 Amps. The best part is getting rid of 60 cycle hum. Best unit on the market. I've been using ART Products for 30 years and have never been disappointed