TorqueL

"Route: Unknown" starts where the normal routes end. To get there you will need to perform a trick at the end of any route. The easiest chambers to do this on are chamber 40 (Route: o), chamber 43 (Route: r) or chamber 44 (Route: T). In these chambers you'll have more room to perform the trick.

Steps to enter "Route: Unknown":

1. Get ready to exit the last chamber of the normal route.
2. Move your character towards the exit using momentum or gravity.
3. Quickly open the menu, note that the game does not pause and your character is still moving.
4. Wait until your character touches the exit, you'll hear a sound.
5. Quickly select "Retry Chamber" in the menu.
6. The exit turns black and will now take you to "Route: Unknown".
7. Beat the chambers of "Route: Unknown" to unlock the achievement, see video below.

If you mess up and enter the exit normally, the game will end and you will have to play the entire route again to get another try. You can back up your save file while you're in the last chamber and restore it to instantly retry for "Route: Unknown".