Guide for the secret ending (spoilers)

The secret ending of the game requires you to get 2 hidden keys, and then using them to open a secret door in the shrine. (Instead of doing the chase-sequence at the end.)

That door in turn leads to a series of platforming challenges, which are harder than the normal game.
Go through it all, and you get the alternate ending!

The required steps:

1. Play normally until you get the extended-dash from the shrine.

2. On the path to the bottom-left Cataract, go here after the falling section, for the first hidden key:

https://i.imgur.com/EoTFw9x.png

3. Go to the second building (where you activate the moving waterfalls), and go in the ceiling for the second hidden key:

https://i.imgur.com/o7wfIXY.png

4. Collect all 3 Cataracts as normal, and go through the shrine. At the end, go to a hidden tunnel on the left wall:

https://i.imgur.com/XQpsBw9.png

5. Continue through this path to the very end. Good luck, you will need it.