Wind blowing through tree branches produces a natural white-noise filtering effect, similar to what you can do with band-reject and band-pass filters, and subtractive synthesis.
This is also what the trekker is asked to do here. The waterfall produces a sound which is very close to white-noise, and it takes only two hands and two ears to modulate a physical subtractive filter.
By opening and closing your ears at different angles, you can transform the sound of the waterfall into different vowel-like timbres, creating your own minimal performance in duo with the water.
The checkpoint is shown on this map: