A LinkedIn blog post about the recent password dump says:
We take the safety and security of our members' accounts seriously. For several years, we have hashed and salted every password in our database, and we have offered protection tools such as email challenges and dual factor authentication. We encourage our members to visit our safety center to learn about enabling two-step verification, and to use strong passwords in order to keep their accounts as safe as possible.
What is the "email challenges" security tool that they mention? I couldn't find anything relevant by searching their website.