Unless the automatic updates are fully disabled it's pushing all the Windows 10 files for the installation (up to 6GB) into an hidden directory:
http://arstechnica.com/information-technology/2015/09/microsoft-is-downloading-windows-10-to-pcs-even-if-you-dont-reserve-a-copy/
After that it should prompt the users to proceed with the installation. Basically for people with a internet traffic cap those are 6GB of bandwidth thrown out of the window. Plus 6GB of disc space, sure.
My thoughts about this: MS isn't famous to give anything out for free. Now they are even trying to force Windows 10. Windows 10 has been defined "as a service". Services are much more profitable than licenses because they usually need a subscription. One must be blind for not seeing where MS is heading to. Just remember the XBL subscription needed to play P2P games (and use the console web browser).







