The projection is made along the "Z" dimension relative to the MapToPlane, that is, in the perpendicular (normal) direction from the MapToPlane surface. This plane may be arbitrarily tipped in space. The point is that the projection is not done in the absolute Z direction, but in the relative Z (normal or perpendicular) to the surface being rubbersheeted. This is referred to as "local coordinates" and differs from the "world coordinate" system used by DX in which [0,0,0] is the origin, X positive is to screen right and Y positive is up as you look down the positive Z axis toward the origin and toward negative Z.