This plugin was designed to be as light and compatible as possible. It doesn't require PHP sessions, a database table or file-based lookups. It uses a bit of cryptography to have the visitor submit all of the information necessary to validate them.
For trackbacks and pingbacks, it ensures that the IP requesting the pingback resolves back to the website it is saying that it is. If so, it retrieves the page that it says contains the link, and makes sure that it does, in-fact have a like to this blog.
The idea for the trackback validation came from the simple-trackback-validation plugin