Several times recently when I have taught my free motion quilting class, I have had students arrive with what (to me) was the wrong presser foot for their machines. I finally went to Quality Sew and Vac and started asking questions, since these students all said they were told by Quality that the foot they brought to class was the right one for free motion.
Lynn Cassidy at Silverdale Quality S&V solved the mystery. Certain models of Viking and Pfaff have a “floating” free motion mode in addition to a “spring action” free motion mode. The machines must be set to operate in one mode or the other, depending upon which presser foot you use. So it is possible to do free motion without a foot with a spring in it, which is the only way I have ever done it.
Here is a link to an explanation by Viking of the Floating Mode:
and a similar link for Spring Action:
Thanks to Lynn for her great explanation of this option available on some sewing machines. Now I will be better able to help my quilting students in the future.