Shows excerpted content for older posts. This allows more items in the feed while working around the Feedburner 512K feed limit.
WordPress feeds offer a choice between Full Text or Summary. The Full Text option is preferable in a lot of cases (e.g. for a podcast), but the feed can grow very large.
One problem with a large feed is that if can exceed the Feedburner 512K limit. Your feed may be valid when you first set it up, but eventually when your feed gets big enough, Feedburner will stop updating.
This plugin gives you another setting (available on your Settings / Reading admin page) which sets how many posts should have Full Text. All posts after that will be shown as Summary. This gives you the best of both options - Full Text for new posts, and Summary for older posts.