It uses scholar as a search engine, but then it replaces the links in the results. e.g.

    http://link.springer.com/link-to-paper

becomes:

    http://link.springer.com.sci-hub.io/link-to-paper

As far as I can tell, it works for some articles.

Just use DOI of the article, then it's instant and bypasses Google search

It's off and on. Last night I could access it but it came back.

US is prob ddosing it...