All I know is that it has a good record. Not of picking the start of a bull market, but of when to start long term accumulation of stocks. It has given both early and late signals by a few months either side of the low point. It has also given false signals - one occurred not long ago on the Dow.
My strong belief is that we should not rely on any mechanical indicator. We must never stop thinking and observing what is happening. The Coppock is not a perfect instrument - it cannot be. So, what is its use?
The reason I teach it is that most people who try to time the market (putting aside the open question of whether this is a good idea) get into bull markets way too late. The Coppock has been reliable enough in the past to be a good wake-up call to these people to start looking to get into the markets.
We still have to find strong stocks. If there aren't any, we are too early, or the signal is false. If we buy some stocks and they fall out of their uptrend, we must act on that signal stock by stock, regardless of what the Coppock says to us. It is not infallible.