Very good question!
I know I may sound a little like Cliff Clavin here but...as a matter of fact there Normy, the trunk drop off is the stamped, sheet metal panel that forms the port and starboard edges of the trunk and then drops down at a 90 degree angle to also form the interior sections of the lower parts of the rearward most portion of quarter panels. ;-)
Here's some of the process:
http://forums.maxperformanceinc.com/...t=drop&page=41
http://forums.maxperformanceinc.com/...t=drop&page=43