How Server-Side Include Work without File Extension?

Question

This question is related to this one,

http://stackoverflow.com/questions/2358178/running-php-without-extension-without-using-mod-rewrite

So by adding this to my .htaccess, I can run PHP script without the extension,

AddHandler server-parsed .php
SetHandler application/x-httpd-php
AddHandler application/x-httpd-php .php

I am not familiar with SSI and I wonder how this magic works? How does Apache httpd know http://example.com/app invoke the script app.php? My real goal is trying to get the extension mapping work without triggering SSI parsing in Apache.