Pfaff Cover Style Owner's Manual page 40

Set the regulator wheel for the differential
feed 41 to 0.5. At this setting the front feed
dog feeds half as fast as the rear feed dog. This
means that the front feed dog feeds only half
as much fabric as the rear feed dog, causing the
fabric to stretch.
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Differential feed
The differential feed consists of two feed dogs
(A + B), one behind the other, which feed the
fabric. The movement of the front feed dog (A)
and thus the amount of material to be fed can
be regulated using the regulator wheel 41. The
setting range is between 0.5 and 2.0.
The standard setting is 1.0, where the move-
ment of the front feed dog (A) is synchronized
with the rear feed dog (B). This means that the
front feed dog feeds the same amount of fabric
as the rear feed dog.
Note:
The function of the differential feed also de-
pends on the set stitch length and the type of
fabric being used.
Here you will avoid gathering/puckering of
the seam when working with very light fabrics,
such as silk and taffeta. Instead, a smooth seam
(A) will be achieved.
You can also achieve decorative effekts with
the differential feed dog. For example the
wavelike effect when using a rib knit (B).