Im not sure if you already know this but, just in case u dont heres a quick explanation (as i understand it, i dont claim to be right ).
Imagine a shelf on the wall, with a lot of little compartments with glass doors on it, with the appropriate keys on them.
~ Deleting a file from your computer (recycle bin) is like taking away the key to that particular compartment with the file in it, The file is still in there, but you cant get to it since there is not key/link! At this point these compartments without keys are by default ready to be overwritten as you delete more files (the traditional way) from ur computer.
~ Overwriting using Recuva jumbles the contents in the compartments, but sometimes whats being over written can be seen through the glass doors (like a file by a picture / some papers with a vase)
~ Wiping free space takes all the contents from all the compartments n chucks em out. its as simple as that
hope ive helped