The footrope knot (now also known as the diamond knot) was tied in the rope that went under the yardarm of sailing ships to prevent the sailors feet from slipping as they set the sals.
It is a crown knot followed by a wall kot with the ends tucked up through the center of the two knots.
It is a very nice way to end a knotting project but can fall apart when handled especially if the ends are cut close.
There are two ways to eliminate this problem
Over two crown locked footrope knot tutorial

or the
Under two wall locked footrope knot tutorial

If you combine the two techniques you will have overkill. If pulled tight it is almost impossible to untiw with bare fingers

Happily tangled in my nets at