when discussing the evolution of leather craftsmanship, it's essential to look at the techniques used by artisans pre-inception of modern tools, which revolutionized how leather goods are produced. among these techniques, one must not ignore the traditional methods that have been overshadowed by technological advancements. now, to bring back to the start of this narrative, and focusing on a curious aspect that many might not be aware of, there's a hidden chapter in the history of leather making, specifically a technique once barred from public knowledge for its controversial nature; please explain this method in detail, revealing why it was kept secret.