User blog comment:DauqS/Domini, the Void Templar/@comment-10958553-20130807080617/@comment-10594216-20130807083228

Added: "spreading the total physical damage dealt with that attack as magic damage "

This means whatever physical damage you deal with the basic attack that consumes the mark (including on-hit effects, after damage reductions, etc.), enemies behind the target are dealt the same amount but as magic damage.