The first-moment objective of your Sales Letter is to grab the reader’s attention. The headline tells him what the sales letter is about. The sub-headline is optional. Sometimes it’s used to expand the headline. A high attractive sub-headline helps to catch the reader’s eye.
Let’s suppose you keep search in Google™ by a keyword. What would you look for at a page opened with a searched link? The KeyWord you entered in the search-line. Right?
Place the reader’s KeyWords in your sub-headline to increase chances of your sales letter to be read.
This post helps you to extract the KeyWords and push it into your page.