The rhythm circuit gives a separate volume and tone setting for the neck pickup, and an overall darker sound, even with the volume and tone controls at maximum. It was designed at a time when "standard" playing technique (at least for the musicians influencing Leo Fender at the time) involved a "softer" and darker tone for rhythm chomping, and a nice, bright, cutting tone for solos. Standard classic country music often works that way, but hardly anybody plays like that anymore, even in country music, though it's actually a useful setup, if you want to follow the classic style, sort-of. I do like the idea of darker, and brighter tones being available for the same pickup(s), though, so I had my Jaguar re-wired so that the rhythm circuit "hears" whatever I select by way of pickup(s) in the lower circuit, giving me a darker, or brighter choice for any selection- whichever one is louder, for "lead", is optional. I also had my two main pickup switches changed so that when they're both in the "down" ( formerly off) position, the pickups are in series with each other, making that another tone for either circuit. I was ambivalent about whether to include the bass-cut switch in what the upper circuit "hears", but right now that's included, and makes a very useful tone when the series option is engaged. I find that my mods give me a really wide range of useful tones for all occasions!