If you subscribe my Blog feed, you may noticed that the feed is not full content output anymore. Yes, I change the way feeds output.
Let me show you how to change it to excerpt in MovableType 5.04.
Then, click save and publish button.
After that, the feed you use is not full content, but excerpt only.
When you write the post, or entry, you can make your own Excerpt. If you leave the Excerpt blank, the program, MovableType will extract the first 40 words from your post as excerpt.