Well, a hook places some jump instruction somewhere in the memory, jump which leads to some code. In my case, I placed some hook at some addresses in the OS which get executed when the Nspire retrieves the XML data.
The good thing is that I did not have to search how the XML is decrypted, but the bad thing is that since I have no idea how to decrypt it, there is no way I can make the same on PC (except in nspireEmu).