When:
During Part 3, right at the very beginning, when you first spawn underwater.
Method:
Walk off screen to the right
Walk off screen to the left
Walk off screen to the right
Walk off screen to the left
Wait a moment for the scene.
Repeat the above three times to unlock your achievement, and a secret premature ending to the game - don't worry, you can reload your auto-save from the start of Part 3.