[ale] Question: What does "Fixating" mean for burning cd?

[tfreeman at intel.digichem.net (tom)]

I grabbed the cd images of Fedora Core 6 yesterday, and I'm happily 
burning my physical cd's now. And a question came to mind which I'm not 
quite sure how to google (yet), so I will also ask the collective wisdom 
of this list.

When burning the cd, cdrecord will announce as the last step it is 
"fixating" the disk. What the heck is physically happening during this 
process? The extent of my understanding of the process is that the image 
file is being placed on the cd medium by melting the bit pattern of the 
image into a spiral track on the cd. The image file already contains 
directory information, so what is happening during "fixating"??