Page 25 - Patrick Moreau, Jean-Benoit Heron Marine Knots How to Tie 40 Essential Knots

The oysterman’s stopper knot is useful when a stopper larger than what is
provided by the simple knots is needed.
Its triangular base acts as an excellent stopper.

1 and 2 - By once again using the same starting movement, first lead the working end twice over and
then under to create an “A” bight twice-under, which you can pull out the top.