After a heavy down day, the next session opens lower still — then closes back above the midpoint of the previous candle. Sellers had control at the open and lost it by the bell.
The second candle must open below the first candle's low and close above the halfway point of the first candle's body, without fully engulfing it. Falling short of the midpoint makes it a weaker on-neck or in-neck formation rather than a piercing pattern.
It is the milder cousin of the bullish engulfing. Buyers recovered most of the previous day's damage but not all of it, so it is best read as evidence that a decline is losing force.
The lower the second candle opens before recovering, the more ground buyers had to make up, and the more meaningful the close becomes.
It fails most often when the gap-down open was caused by overnight news, and the intraday recovery is simply the market repricing an initial overreaction rather than any change in trend.
A recovery that stops just above the midpoint is barely distinguishable from an ordinary volatile session. Treat marginal cases as noise rather than signal.
As with all two-candle reversals, the following session does the confirming. A stock that gives back the piercing candle's gains the next day was never reversing.