It's as simple as the idea of using resin is to "penetrate (hahah) or saturate" the paper from the outside so it will not distort or warp the pep model when slushing SmoothCast or Fiberglassing on the inside of the helm. Obsiously Resin will not soak thru the foil. So if you line the edges of the adjoining faces (i.e. the faceplate, & jaw to the main body of the helm) with foil then you will be able to have 3 easily seperable pieces once the reinforment is complete vs having to cut & dremel the pieces apart.
Here's how foil is used in the stereo world when doing a FG sub box say.....You was to make a mold for a box of a spare tire well....You put foil down & press it to the contour of the panel you want to mold off of, then you apply masking tape to hold the foil to the shape. Then you start to lay your Fiberglass Mat and Resin.....Once cured the foil will easily allow your mold to release from the "Master or Mother" it was made off of. Hope that helps a little....