You will need two keys and two locked treasure chests.
As you open the first chest hold down the Help key. Having collected the treasure, open the second chest while holding down the Insert key.
An ankh (symbol of life) should be revealed. Collect this and a gong will sound and you should now have 50,000 health points and 32,000 reflective super shots.
This cheat mode can be reactivated at any time. You will lose the super shots when you go to another level.
(can someone get a screenshot of this in action?)