Google introduced “On-Demand Indexing” in Custom Search for faster crawling and indexing web pages. Rajat explained the steps required: Just go to CSE control panel, click on Indexing tab, select On-Demand Sitemap, and hit "Index Now" button. Then tell which of the URLs are most important to you via the priority and lastmod attributes in your Sitemap. Each CSE has a number of pages allocated within the On-Demand Index, and with these attributes, you can tell Google which ‘re most important for indexing. If you need greater allocation in the On-Demand index, as well as more customization controls, Google Site Search provides a range of options.

Some important points:

1. Submit once your Sitemaps in Webmaster Tools and select Sitemap for On-Demand Indexing.
2. Your Sitemap needs to be of verified website in Webmaster Tools.
3. In order to index these additional pages, Google’s crawlers must be able to crawl them. You can use "Webmaster Tools > Crawl Errors > URLs restricted by robots.txt" or check your robots.txt file to ensure that you're not blocking us from crawling these pages.
4. Submitting pages for On-Demand Indexing will not make them appear any faster in the main Google index, or impact ranking on Google.com.