I'm not sure that always works.
Didn't work for me anyway. There are three other ways to survive the trap...
Go just over the edge and backup until you are just on the edge. Kneel, lean forward, and grab the crown. You'll get webbed but you can get rid of them safely.
Stay near the edge but on the sinking platform. Grab the crown. While webbed, quickly turn around and mantle backup onto the edge.
Just grab the crown. Remove the webs on the way down. When you get to the bottom there will be a little tunnel you can escape through. You may take a little damage this way from the gas though.