Switch between guitars or amps with just a tap. The switch on the pedal gives you the option of "one or the other" or "both" meaning you can play on two amps at the same time, or one at a time, and the same with guitars. Relatively cheap and a good pedal to have if you're playing with multiple amps or guitars in different tunings.
You can do a pass-through to two amps, or you can plug-in two guitars and with a 'tap' choose one or the other. This helped me a lot. Because I always have to change guitars and this is a real time saver! This is something I miss when I use the Boss ME-80 multi-effects.