I think they are referring to the little black button on some of the feet that you can press at the edge of the fabric so the layers don't slide and the foot will stay horizontal.
With the 6600 you should be using the Acufeed anyway. I had the 6600 and thought it sucked using the regular 1/4 foot, hated that machine, so I traded it in for the 7700, same problem, until they came out with the redesigned 1/4 foot for the 7700. It has a little indent underneath that makes the whole difference.
You might want to get the redesigned foot for the 7700 to use on your 6600 if you don't want to utilize the Acufeed for the bulky stuff.