Vincent implementation of OpenglES 1.1

Dear All,
I am going through Vincent opengl es implementation. My query is about pixel rasterization implementation.
Having the vertices & color values at 3 corners of a triangle, deltaX,deltaY for color along edges 1-2 & 1-3 are derived as follows:
vars.VaryingInvW[index].dX = Mul64(det2x2(DY12, IVW12, DY31, IVW31), invArea, 28);
vars.VaryingInvW[index].dY = -Mul64(det2x2(DX12, IVW12, DX31, IVW31), invArea, 28);

Could anybody please explain me, how does this exactly works.? What is the significance of finding determinant hereā€¦?

Best Regards