The cheat mode is 'FRANK' backwards and it must be entered during play as follows:
Press and hold 'K', press and hold 'N' while holding 'K'.
Release 'K', press and hold 'A' while holding 'N'.
Release 'N', press and hold 'R' while holding 'A'.
Release 'A', press and hold 'F' while holding 'R'.
Release 'R', release 'F'.
You are now invulnerable to enemy fire. At the beginning of each level the cheat is disabled and must be reentered to reenable it.