Only time this rule failed was after Mach 2020
1. Markets tend to return to the mean over time.
Translation: Trends that get overextended in one direction or another return to their long-term average. Even during a strong uptrend or strong downtrend, prices often move back (revert) to a long-term moving average. The chart below shows the S&P 500 over a 15-year period with a 52-week exponential moving average. The blue arrows show several reversions back to this moving average in both uptrends and downtrends. The indicator window shows the Percent Price Oscillator (1,52,1) reverting back to the zero line.
2. Excesses in one direction will lead to an opposite excess in the other direction.
Translation: Markets that overshoot on the upside will also overshoot on the downside, kind of like a pendulum. The further it swings to one side, the further it rebounds to the other side. The chart below shows the Nasdaq bubble in 1999 and the Percent Price Oscillator (52,1,1) moving above 40%. This means the Nasdaq was over 40% above its 52-week moving average and way overextended. This excess gave way to a similar excess when the Nasdaq plunged in 2000-2001 and the Percent Price Oscillator moved below -40%.