PE exposure from the base ,with yellow dye inhibiting exposure further "up" is probably why it works as a matrix film ? With more light ,proportional to the negative density,penetrating further and yeilding higher areas after hardening and clearing.
That's right Bill, the yellow dye only works for a blue-sensitive (maybe ortho too) emulsion and is specifically useful for exposing through the base and creating a "tenuous" relief. Too high a relief and you get trouble maintaining flush contact between the matrix and the receiving paper. F.E. Ives first patented this idea and used in for dichromated-gelatin (carbon) matrices. Pan-matrix film used a black dye for the same purpose.