This beautiful piece by John Propitius began as an improvisation, and is clearly in that style. It has since been performed numerous times by him for various concerts and CDs. I recently transcribed this Fantasie from this video: https://www.youtube.com/watch?v=KVf5XcLK688. It strongly utilizes a playing technique widely used among Dutch organists and composers which has been nicknamed by @Jerry Martin "playing the wind." This creates a thrilling and unique effect as the pipes detune slightly when used, due to the unsteady wind pressure.
If anyone is interested in the score, leave a comment letting me know or send me an email and I'll gladly send it over.