Spoiler Warning
This page explains Endacopia’s all-endings secret. Finish Ending A, Ending B, and Ending C first if you want to discover this scene without knowing its setup.
Direct Answer
Complete Ending A, Ending B, and Ending C. After the three main endings are registered, an additional scene appears with Andyl4nd at a computer. Speak to him, start the song, and complete the dance minigame by pressing the arrows in time to unlock The Yeti achievement.
Unlock the All-Endings Scene
The Yeti route is not a separate Chapter II branch. It is an additional scene that appears after you have finished all three main endings. Check that each ending has reached its own completion sequence before looking for the extra scene.
If you are missing one ending, use the save before the Chapter II saw choice to return for Ending A or B. For Ending C, return to a Chapter I save with the three area secrets prepared; it cannot be substituted with either normal saw route.
Meet Andyl4nd
Once the game recognizes the three main endings, look for the scene with Andyl4nd at the computer. Talk through the dialogue and accept the offer to hear the song. This begins the final dance sequence.
The scene is a reward for all-ending completion, so do not look for a hidden item or alternate saw choice to trigger it. If it does not appear, recheck that Ending A, Ending B, and Ending C were each completed on your current game data.
Complete the Dance Minigame
During the song, press the directional arrows when they line up with the beat. Focus on timing rather than rushing through the inputs; the dance sequence is the last requirement for the achievement.
When the minigame finishes, the game awards The Yeti achievement. This is the final completion check for the route.