He starts by talking about what the library does to backport the code from PHP 7 to PHP 5 and how to install/use the command line tool. He then gets into things "behind the curtains" with a sample PHP 7 class and the resulting PHP 5 code. They made use of the PHP Parser tool for processing the PHP code provided. It does matching on certain element types and performs the token replacement. He gives examples of this with the null coalesce operator replacement and scalar type hinting.