The Psychology
In an uptrend or within a bounce of a downtrend a long white day occurs. The next day's gap down comes as a surprise to bulls who thought they were sitting on a great position the previous day. Reliability of the bearish Harami is low, so a weak following day is needed for confirmation.

A long white day is followed by a black day that gaps down and is completely engulfed by the real body of the first day.

Pattern: reversal
Reliability: low