Probably cookies and javascript. Lots of sites would like to make the paywalls immediate, but they won't get ranked highly on search engines if they don't show

Probably cookies and javascript. Lots of sites would like to make the paywalls immediate, but they won't get ranked highly on search engines if they don't show the article's content to a web crawler. So the usual approach is to load the content, then hide it as quickly as possible if an unauthorized human is viewing the page.