Principle number 3:

The study of the price charts for many decades revealed that certain patterns appear frequently, indicating bullish or bearish sentiment.
These patterns are based on human psychology that tends to remain the same. Fear and greed leave their trails behind so we can forecast their next move.
So, identifying a Head and Shoulder pattern after a prolonged uptrend, I would expect a reversal to the downside. Do I really care what causes the reversal? The comments are yours.

Similarly, the presence of an Inverse Head and Shoulders after a prolonged downtrend will signal a reversal to the upside.
These patterns worked fairly well in the past, and the technical analysts expect them to work equally well in the future.
After all, technical analysis studies human psychology.