A long black day is followed by a small day that gaps in the direction of the trend. The third day is a white day which closes in the top half of the black day.
In a downtrend or during a pullback within an uptrend, the market gaps down but enough buyers step in to halt the weakness. The lack of ability of the bears to press the issue indicates the downtrend may be weakening. The gap up and rally that closes the white day above the top half of the black day confirms the reversal if accomplished with a surge in volume.