Has anyone here heard of a contact printing technique in which a sheet of opaque white plastic is lain between the film being printed and the glass plate which is weighing down the film.

I have heard that it was a diffusion technique used in Ilfochrome / Cibachrome contact printing and have a hunch it was to help with issues in contrast? but this is just a hunch. Has anyone gotten experience with this Contact printing methodology? what plastic sheeting is used? and what is the true reason behind this method?

~Steve
The Lighthouse