In simple terms, ROMPatcher allows you to apply "patches" to the phone's Read-Only Memory (ROM) while the phone is running. It bypasses the security checks that Nokia and Symbian built into the OS.
If your phone becomes unstable after patching, you can use the Hard Reset code *#7370# (default security code is 12345 ) to restore factory settings. Note that this erases all personal data . Nokia-6600-apps-s60v2-rompatcher