[reeckoh]

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.

[midoridensha]

Exactly, which is why extensions like Bypass Paywalls Clean can easily bypass these paywalls just by changing some Javascript.