The Zig Zag indicator is used primarily to help you see changes by punctuating the most 
significant reversals.
It is very important to understand that the last "leg" displayed in a Zig Zag chart can 
change based on changes in the underlying plot (e.g., prices).  This is the only indicator 
in this book where a change in the security's price can change a previous value of the 
indicator.  Since the Zig Zag indicator can adjust its values based on subsequent changes 
in the underlying plot, it has perfect hindsight into what prices have done.  Please don't 
try to create a trading system based on the Zig Zag indicator--its hindsight is much better 
than its foresight!
In addition to identifying significant prices reversals, the Zig Zag indicator is also 
useful when doing Elliot Wave counts.
For additional information on the Zig Zag indicator, refer to 
Filtered Waves by Arthur Merrill.