Rugs, Carpets and Carpeting - HArdwood Protection or Unusual SubFloor?

I was told we have hardwood floors under the wall-to-wall carpeting in two rooms. When I pulled up a corner of the carpet and pad, I found neither hardwood floors nor the usual plywood subfloor. Instead, I found what appears to be a white particle board. Someone told me this protective layer was put down to protect the hardwood floors from the nails of the tack strips. I've never heard of this before and was wondering if anyone out there knows anything about such a protective layer. Thanks in advance.