To the best of my knowledge, it works the same way as the COPPA registrations, where if an underage person tries to re-register with another birthdate - they will not be allowed to register. A cookie that is not removed when cookies cleared is set. If user tries to register again on that computer - they will be blocked. I haven't actually used this plugin yet, but am considering it for the future.
You can block by prevent multiple registrations from same IP, but I have found that many of these are genuine registrations where two or more people are using the same computer. I check the IP of new registrations, and if there is another registration from the same IP, I monitor new posts to see if registration seems to be genuine. If not - I ban.