The serrated drive attaches to the blade and drives the blender. These are poorly designed rubber thingies and need to be replaced often. I have done this before, but now I can't remember how I did it.
A picture of this part can be found at:
http://www.abtecparts.com/commerce/search/products/?product_id=09XKR0698195&merchant_id=1933

Callorhinus, June 2005

I just took a couple of pairs of pliers to rip off the old rubber and then a hack saw to cut through the nut. I couldn't see a way to open the base of the blender. Anyway, it worked like a charm! The new piece screwed on nicely.

Rich, July 2008

I have a different model, but the replacement part is the same. I needed to open the bottom of the unit, hold the shaft of the electric motor still with a screwdriver and just turn the rubber part loose.

Good luck!

Antti, August 2005