Complex sentence: Using "it" instead of the whole clause "what you said" is called substitution, or replacement one of the tests used to determine the internal structure of a sentence, i.e. to determine its constituents.
Split into simpler sentences: Using "it" instead of the whole clause "what you said" is called substitution, or replacement. This is one of the tests used to determine the internal structure of a sentence, i.e. to determine its constituents.