Just imagine correlating prices with a straight line having a positive slope. If the price trend is up, the correlation is nearly +1. If the price trend is down, there is anticorrelation and the correlation is nearly -1. If the price trend is sideways, or if prices are oscillating, there is no correlation over the span of the correlation period.
This is a companion discussion topic for the original entry at https://www.optuma.com/kb/optuma/tools/ehlers/correlation-trend