You modify a version of the original firmware and it just boots instead of the original, much like a computer with two operating systems. You download an original version of the firmware, mod. it, and then load it. As soon as you turn it on, it loads the Rockbox version. Check the other Rockbox threads, I have posted links to how to load the Rockbox firmware and also the "optimized" firmware which has all of the latest recording enhancements and should do negative gain. Gain is analog up to around +20db and then goes to digital gain. The gain is labeled to show you exactly where it goes from analog to digital as well. Have fun!