Here's a great close-up (from Bob Kurasawe) of the swollen grain one gets when soaking hides in alkali (wood ashes, Red Devil lye, or hydrated lime). The section on the left has the grain intact, on the right it is removed.
If you use good technique and are careful to scrape each area thoroughly before moving on, you get a good crisp transition line like this. It makes it very obvious what still needs scraping, and it makes it easier to do.