Brother PE 400D - Manual (Page 36)

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The feed dog adjustment lever can be seen at the back of the
sewing machine. Sliding the lever to the left will lower the feed
dogs, for example during button sewing.
If you wish to continue sewing normally, slide the lever to the right
in order to raise the feed dogs.
The feed dogs cannot be raised if the balance wheel does not
turn, even if the lever is slid to the right.
FEED DOGS