A long tail is when there are ‘1’ markings in the last column of the BoP on the bullish side or bearish side and no other 1’s in any of the other columns. This typically marks the end of the bullish or bearish herding, especially if there’s been trending for an extended period of time.
Below is an example from a bullish BoP (period shown: 1/4/18-1/26/18).
The trend changed at the end of the ‘long tail’ into a bearish herding trend:
Note that at the end of a ‘long tail’, just before the trend changed, there was no new herding in the 1st to 5th column of the BoP.