Complex sentence: When one uses Nilquest subsampling as a standards conversion technique, the horizontal and vertical resolution of the material are reduced--this is an excellent method for converting HDTV to standard definition television, but it works very poorly in reverse.
Split into simpler sentences: When one uses Nilquest subsampling as a standards conversion technique the horizontal and vertical resolution of material are lowered. This is an excellent method for HDTV conversion to standard definition television--but it works very poorly in reverse.