A long white day is followed by a small body that gaps in the direction of the trend. The third day is a black day that closes in the bottom half of the white candle.
In an uptrend or within a bounce of a downtrend, the market gaps up but does not continue its upward movement. Instead enough bears step up to bring supply and demand back into equilibrium so a small body forms. A subsequent follow through gap down that closes below the midpoint of the white day completes the pattern and confirms the reversal.