Stage 1 Invulnerability:

At the very start of stage 1, at the top of the train, wait for the timer to go 87 seconds. When you hear a "kan kan" or "tan tan"-like sound (2 quick high notes), throw a dagger on your left. Do this 4 times. The invulnerability will be activated when a bonus score is displayed. The invulnerability lasts only for stage 1.