Facility score may IMHO be understood as some statistical measure of tactical involvement. It has really nothing to do with the established and important concep

Facility score may IMHO be understood as some statistical measure of tactical involvement. It has really nothing to do with the established and important concept of weakness in chess, which looks at significant static features of a position. For example in the position from figure 1, white's pawns on the queen side are all weak, especially c4. Also the black squares around the white king are weak. After 1...exd4 these weaknesses are pronounced sharply. Considering fragility score, I see nothing that points towards these weaknesses. a4 and b6 are not attacked, hence ignored. c4 is attacked once and defended once, but there is no trajectory over it. Weak squares are not considered. If we look at the knight D4, it's BC score may tell us, that after taking it with a pawn, which can be done in two different ways, the bishop e3 is hanging, hence we gain a tempo, which is significant, also f3 is weaker (noise) and bishop e6 is not under pressure any longer. Hardly something which convinces to identify the key moment, but may be useful in a statistical sense. Nevertheless it appears to be an oversold complexity measure.