[Q] CF-Auto-Root for Nexus 5 - How it works? topic

Hey guys,

I couldn't find it anywhere and I don't really know if this is the right place to ask, but I'll give it a try...
I wonder how does the CF-Auto-Root for the nexus 5 works?
I can see in the windows batch file that it unlocks the bootloader (that's the easy part) and than boot with some image file.
It seems that this tool is not installing any custom recovery which I always saw is a necessary tool for rooting.
What exactly is this image file? what does it do? Where does it come from? What it contains?
Why it's device related (different image files for different nexus devices running the same stock version).

Thanks,
Casteel.