A message will confirm the correct code entry.

Invincibility:
Press Back and then press X, X, A, B, A during game play.
Your first soldier will be invincible, but the remaining members of your platoon
can still be harmed.
You cannot complete the game normally when this code is active, as maps and
dossier items can no longer be unlocked.

Big heads:
Press Back then press A, X, B, Y, A during game play.

Chicken explosives:
Press Back then press X, X, Y, A, B during game play.

Paper mode:
Press Back then press B, A, X, Y, A during game play.
Characters will be two dimensional in this mode.

Slow motion:
Press Back then press Y, Y, B, X, A during game play.

High pitched voices:
Press Back then press X, A, Y, B, X during game play.