To use this, you must first active god mode in any difficulty level except Damn, I'm good. After that, wrap to a boss and kill it. Once you have done that, start a new game on Damn, I'm good and you'll be in god mode. This works until you go to a lower difficulty level and deactivate it or quit the game entirely.
This is an extremely easy glitch to do, but only works in the original Duke Nukem 3D for DOS and variants.(Plutonium Pak, etc.) Simply select Weapon 1(Duke's Foot) then hold the Fire button. As you're doing this, hold down the Mighty Foot button as well. Duke is now kicking 1.5 times per kick! Not only that, but he's doing so using both his left and right feet at the same time. How gravity defying.