An a/b switch has a single input and two outputs, a footswitch will divert the input signal to either of the two outputs off the pedal. Here's one for example:
http://www.elderly.com/new_instruments/items/270.htmYour LC-30 has a parallel fx loop with a send (output) and return (input) on the back panel. The signal path you need is [send] -> [a/b switch] -> [return]. One of the a/b switches outputs remains unused. This gives you two possibilities:
1) the A/B switch blocks the signal (which is diverted to the unused output) so nothing happens, your amp works as usual.
2) The A/B switch connects both cables so the loop is closed. Now, if you turn up the effects dial on the front panel you will notice the sound will get louder the more you open it up. This is your solo boost sound. It works on both the clean and overdrive channel.
You can test this without an a/b switch by using a patchcord plugged into the send and return. Turning up the effects dial will make your amp louder, somewhat like a second master volume.