The ipod power-on behaviour is the following:
1) Like all ARM processors, the code at address 0x0 is executed when the ipod is powered on. On the ipods, this is the flash ROM, and contains the Apple bootloader, disk-mode, diagnostics mode and (on the 5g), some firmware which is transferred to the Broadcom processor.
2) After initialising the hardware, the Apple bootloader checks for the key combinations to enter either disk mode or diagnostics mode, and if they aren't being pressed, it will load the main Apple firmware from the firmware partition. The firmware partition is the first partition on the disk, and is marked as type 0 (Empty). This prevents operating systems like Windows/Mac OS X/Linux from attempting to mount it.
3) ipodpatcher attaches the Rockbox bootloader to the end of the Apple firmware in the firmware partition and changes the header information in the firmware partition so that the Apple bootloader will load both the Apple firmware and Rockbox bootloader into RAM, but execute the Rockbox bootloader.
4) The Rockbox bootloader then either loads and executes the main Rockbox firmware (the rockbox.ipod file on the FAT32 partition) or executes the Apple firmware already loaded to RAM by the Apple bootloader, depending on whether MENU/Hold is pressed.