This method was brought to my attention today by one of my facebook followers.
A more indepth explanation is located on mozfiberlife.wordpress.com
Here is a brief explanation:
The basis of the science behind this, is the concentration of the potassium salts from the fatty acids excreted by the suint glands. The suint is alkali in makeup and combines with the fats in the fleece to make a soap. The mention of rain water is very important. Rainwater is soft and soaps works best in soft water. The drawback to this method is that it has a considerable soak time and depending on the greasiness of the fleece, you may still have to do a hot water/detergent wash.