All the movement commands will try to go to the previous (or next) article, even if that article isn't displayed in the Summary buffer if this variable is non-nil. Gnus will then fetch the article from the server and display it in the article buffer.

This hook is called whenever an article is selected. By default it exposes any threads hidden under the selected article.

This hook is called whenever an article is selected. It is intended to be used for marking articles as read. The default value is gnus-summary-mark-read-and-unread-as-read, and will change the mark of almost any article you read to gnus-unread-mark. The only articles not affected by this function are ticked, dormant, and expirable articles. If you'd instead like to just have unread articles marked as read, you can use gnus-summary-mark-unread-as-read instead. It will leave marks like gnus-low-score-mark, gnus-del-mark (and so on) alone.