Complex sentence: A major stumbling block for many market timers is a phenomenon called "curve fitting", which states that given set of trading rules has been over-optimized to fit the particular dataset for which it has been back-tested.
Split into simpler sentences: A major stumbling block for many market timers is the phenomenon of curve fitting. This means that a given set of trading rules has been over-optimized to fit the particular dataset for which it has been back-tested.