pre_system hook

[eluser]jonyr[/eluser]
Hi everybody, congratulations for this excellent forum. There are here very interesting information.
I'm very new in codeigniter, so I reading a lot because I love this framework.
I have the next question:

Scenario:

---- /application/config/config.php -----
$config['enable_hooks'] = TRUE;

---- /application/config/hooks.php ----
$hook['pre_controller'] = array(
'class' => 'my_cache',
'function' => 'pre_cache',
'filename' => 'my_cache_hook.php',
'filepath' => 'hooks'
);

How many times this function (pre_cache) will be executed, if I call this url http://mydomain.com/index.php/welcome/index ? (It is the default controller)

To check this, in the constructor of my_cache class, I'm creating a file with a random name using file_put_contents. Something like this
-----------------------------------------------------------
$file_name = microtime(true) . ".temp";
file_put_contents($some_path . $file_name, microtime(true);
-----------------------------------------------------------

I am a bit confuse, Why two of this files are created?

Could anyone help me to understand this?

Thanks in advance.

JonyR