A stopper knot is put in the end of a line,
to form a positive mechanical stop against the tail pulling through. A
constrictor, Yosemite tie off on a bowline etc. achieve this by sitting on
their own tail, after a secure knot is made. This is serious stuff,
failsafe should always have at least 2 independently operating security
systems to maintain positive check. My G.I. Jane Math: "Your going to war,
always have a backup.....2 is 1 and 1 is None!". Lock your open hitchings,
especially on the open lifeline systems!!
Knots are pictured loose,
always Tie, Dress, Set (and Inspect) your knot lacings.