Sorry to be pedantic, but...
Are the three occurrences of 'when' in the intro meant to be 'then'? The sentence would make more sense with 'then'.
'Pawns may promote any piece, used here, expect king.' As written, this is not fully explained. When does promotion take place and what, for example, does a Pawn promote a Bishop to? Or do you mean 'Pawns may be promoted to any piece, used here, expect king.' - in which case it would be understood that this happens on the far rank unless otherwise stated?