The piercing line is a two-candle bottom reversal: a large bearish candle followed by a bullish candle that opens below the prior candle's low but closes more than 50% into the first candle's body.
Bulls aggressively counter at the lows. Despite opening lower, buyers drove price up strongly, closing deep into bear territory. An open-low-close-high signals accumulating demand.
A bottom warning signal after a downtrend (win rate 64%). Consider entering long after the bullish candle closes. Stop-loss below the second candle's low. Deeper penetration (>70%) = stronger signal.
The bullish close must penetrate more than 50% into the bearish body. Less than 50% is indecision only.