The psychology behind a harami is momentum exhaustion, where the dominant side loses conviction and the market pauses before potentially reversing.
When the smaller candle appears, it signals a pause. Momentum is fading, and the side that was driving the market is no longer as confident, which is why the harami is read as a reversal pattern.
In a bullish Harami, the initial long red candle reflects strong bearish momentum, but the small green candle that follows shows indecision; buyers haven't taken control yet, but sellers are losing steam.
In a bearish Harami it's the opposite, where bulls are in charge until the smaller red candle shows the rally is running out of steam.