Activating this trainer:
Press F1 at main menu.
Listen for 'Trainer Activated'.
Press desired hotkey. All hotkeys can be changed on the trainer.

Trainer Options:
NumPad1: Infinite Health
NumPad2: Unlimited Jumps
NumPad3: Fly / Ghost Mode
NumPad4: Unlimited Ammo
NumPad5: Rapid Fire
NumPad6: Untouchable

Notes:
Unlimited Jumps: While active you can jump unlimited times.

Fly / Ghost Mode: While active you are invincible and can fly around, even through objects, using the movement keys.

Untouchable: While active you are not only invincible but also everything goes 'through you'.