PageRank sculpting is the act of optimizing your pages to flow PageRank from your highest PR pages through to your most important internal pages while shunting PR from unimportant pages by directing Google to no longer crawl or index those unimportant or irrelevant pages. Why do you WANT Google to start ignoring some of your pages? Because as you flow your high PR through to only your other valuable Pages you increase the PR on those pages and as you drop the pages that have no relevant content or PR from your site- you leave Google with only high quality, high PR pages. Starting to see the value yet? You have now increased the ratio of high PR pages on your site and look like an overall more authoritative and relevant site. Basically, we have stopped Google from wasting its time with our loser pages and kept it circling our important pages.