Pfaff coverlock 4852 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
movement 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
depends 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).