From Traderpedia

This is a formation made up of two candlesticks and is found at the bottom of a downtrend. The Piercing Pattern will ususally signal a reversal and is more powerful when found at a level of support.

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The formation of a Piercing Pattern commences just as it would for a Bullish Engulfing formation with the second candle opening lower than the close of the first candle - signalling that the bears are still in charge of the market.