On mention of installing the online voter registration tool from ROCK THE VOTE, someone asked “How do they know if the person signing up is a U.S. citizens?”
I had not tried it, but later captured this screenshot from the page three of the form.
Instructions for completing Rock The Vote’s online registration
It seems that—since the online tool merely produces a registration form (PDF), to be printed out and mailed to a Secretary of State—it does not check the identity or citizenship of the individual. That is left to the state to verify.