Type / and one of the following codes during gameplay: 

Code        Effect
------------------------------------
CLOAK     - turns cloking on/off 
EYE       - changes view angle 
IRANGE    - unlimited range of firing 
IRONMAN   - invincibility 
REGENALL  - regenerates everything 
REGENDAM  - regenerates health 
REGENWEAP - regenerates weaponry 
STAGEWIN  - to complete the current stage