Was able to log in despite that, though. I don't think it's the exact same thing as the e-mail authentication that Mif is talking about.
What I saw in the stress test was asking me to verify "Hey, is this your e-mail?"
Mif's quote suggests that if you log in from a computer that you don't normally log in from, you will get an e-mail going "Hey, someone's trying to log in on your account, is it you?"