If I understand correctly, the performance mode puts the processor constantly at 400 mhz, while "on-demand" lets the processor scale its performance up and down, due to what is required of it and to save battery. If it's not being used, it can operate, say, at 200 mhz and it won't impact use. The "conservative" mode keeps the processor lower than the maximum at all times. The processor is never "overclocked". To keep the device and components safe from overheating, the processors is never forced to go to 600 mhz or 1 ghz.