Brother T-8421D Instruction Manual page 51

Setting details
Automatic sewing is not carried out, and sewing stops when the treadle is
returned to the neutral position.
Sewing is carried out automatically for the number of stitches which have been
set (start and end backtacking and fixed stitch sewing) simply by depressing the
treadle once.
After sewing is complete, depress the treadle backward to carry out thread
trimming.
Sewing is carried out automatically for the number of stitches which have been
set (start and end backtacking, fixed stitch sewing, thread trimming) simply by
depressing the treadle once.
After sewing is complete, thread trimming is carried out automatically.
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