How Improved Registration works?
Improved Registration can hide "repeat email" and "repeat password" fields, and users can register by email address instead to use username.
Also Improved Registration can create a blacklist of usernames and emails to not let them to register to the website. Of course whitelist can be used instead of blacklist.
You can try to register here, for example, by an email address from "gmail.com". You will not be allowed to continue the registration because that email address is in the blacklist.