Earth is the source of Metal. Metal melts and become like Water. Water in turn helps Wood to grow. Each element in this cycle produces the succeeding element. The destructive cycle - Wood destroys Earth by getting nutrients from it.
Fire destroys metal by melting it.
Earth destroys Water by blocking it. Each element in this cycle destroys the succeeding element. The reduction cycle - Wood reduces Water because it used up water for its growth. Water reduces Metal because metal is used up to produce water-like liquid. Metal reduces Earth because earth is exhausted in the process of producing metal.