This footswitch allows you to engage and disengage the fat switch on your Fender Blues Junior. There is no light to tell you if it's on or off, so you need to listen or remember, although either way that is fairly easy to do! The Fat Switch does pretty much what it says - adds fatness, and, to my ears, some mid-boost. I think it is slightly over-priced for what it is, but it does look good, and it is very handy indeed if you're playing live. However, as the Blues Junior costs around £465, I think it should perhaps be included with the amp as standard.