They are, what GP is describing is called a slipped reef/square knot. Reef knots without the loop can also be capsized for quick untying. It is a dangerous pro

They are, what GP is describing is called a slipped reef/square knot. Reef knots without the loop can also be capsized for quick untying. It is a dangerous property of that knot for some situations, and a reason some people think is shouldn't be taught as a general purpose knot. But it can be useful in specific cases like in reefing, as its name suggests.