It's a meditation technique.
Sit down into seiza or lotus (half-lotus also works) position, inhale with nose for 2 seconds, exhale with mouth for 8 seconds. Adjust the breathing.
Then imagine yourself being in the center of a river, but remaining in our positions and unscathed by the flowing water. Imagine the water as all the thoughts, that are flowing in your mind. Every voice, thought, dream, EVERYTHING flows past you into an empty void, where it is destroyed.
That's it.