After defeating Drek in main game mode, pay a visit to the "Goodies" menu where you will need to complete one of the following combos.

Big Head Mode (Ratchet) : Press Flip Back (x3), Full Second Crouch, Stretch Jump, Full Second Glide.

Big Head Mode (Clank) : Press Flip Back, Hyper Strike, Comet Strike, Double Jump, Hyper Strike, Flip Left, Flip Right, Full Second Crouch.

Temporary Invincibility Whenever Health is Full : Press Comet Strike (x4), Flip Back, Full Second Crouch, Flip Back, Full Second Crouch, Comet Strike (x4).