Rotary Drum Filters
A rotary drum filter (RDF) is a mechanical filter which removes waste and other fine particles from the pond water via a micro screen (filter screen). What makes the RDF so special is the ability to "monitor" the changes in water levels and "recognize" if the micro screen needs to be cleaned. Therefore a rotary drum filter automatically adjusts to the waste intake and only flushes if necessary. The other big advantage of RDF's is that all detritus such as fish waste, uneaten koi food etc. would not remain in the pond water. Instead virtually all waste is flushed from the water column automatically after a very short period of time.
In common filtration systems (such as multi chamber systems with settlement or vortex chambers etc.) the waste would not get back into the pond but it still remains in the water system until the next backwash. During that time hazardous substances such as ammonium/ammonia etc. are still diluted into the water. Due to the frequent removal of waste by an RDF less ammonium/ammonia is contaminating the water and therefore the filter bacteria in the bio stage of your filter will work much more efficient. The advantage is that the space requirement for the filter area is significantly reduced (a good point if the garden does not leave a lot of space for building a koi pond).