When you are knocked to the floor, pause the game (space) and unpause again (space). You will now be invulnerable. Be careful!, this works for the computer, too!
Codes to type in while playing:
"FREZ" - Freeze the game
Other words that cause effects are:
"ANBK" | "ANGL" | "EDHK" | "FOOK" | "GLZP" | "GPZP"
"SHAH" | "SIMR" | "STEW" | "SUNL" | "TOTO"
Typing "FUCK" and "CUNT" results in an interesting message.