Part 1

1. go to Admin Console
2. go to Website & Content → Structure & Data section
3. go to News category (on the left)
4. switch to Articles tab (on the right)

Part 2

1. open new article creation page by clicking on New Article toolbar button
2. confirm, that Generate from the article body checkbox is unchecked
3. enter something into Title & Article Body fields, but not in the Article Excerpt field
4. press Save toolbar button
5. open created article for editing
6. confirm, that Generate from the article body checkbox is unchecked
7. confirm, that Article Excerpt field isn't empty

Part 3

1. open article created previous for editing by selecting it in grid and pressing Edit toolbar button
2. change contents of Article Exerpt field
3. press Save toolbar button
4. open created article for editing
5. confirm, that Article Excerpt field value isn't changed (user entered value stayed)

Part 4

1. open article created previous for editing by selecting it in grid and pressing Edit toolbar button
2. check Generate from the article body checkbox
3. press Save toolbar button
4. open created article for editing
5. confirm, that Article Excerpt field value was regenerated (user entered value was replaced by auto-generated one)