Storage API stream wrappers Traffic Usage

Hi all
In my Drupal 7 Project I have configured Storage API stream wrappers in order to save all public and private files over ftp server.

everything works well and files in public and private are copied into ftp server correctly.

Imagine that my base server is domain.com and my ftp server is ftp.com

As you know, public files will be shown with direct link to the source for example

http://ftpdomain.com/public-file.png

and private files are not shown with direct links for example

http://domain.com/system/storage/serve/private-file.rar

Now my question is: when a user try to download private-file.rar via above link, what happens really?
is private file from ftp server being copied to the main server temp folder at first and then download starts?
Or private-file.rar is downloaded directly from ftp server to the client machine despite the URL starts with "http://domain.com/.... "

It is very important for me because of shared server rules in file download limitations , I have to purchase one **PHP host** for running Drupal script and one **download host** for placing files folder in order to run a Drupal project commerce file shop.

I want to be sure, download traffic only relate to download server not PHP server.

Thanks a lot for your guidance.