Is that just a knot to prevent the prussic knot from sliding off?
Not on mine..Somerimes I go without that Plussiax knot..I simply stand up and put my big loop rope as high as it will go..and pull tightly..

Finally, what type/size rope do you use?
The rope rells you what it's strength it is on the package..Mine is at least 500 lb strengh.I currently have a soft nylon rope, picked for its suppleness, but I am a bit concerned with the 'stretch' that it has, is a stiffer rope preferred even though it doesn't move as well?
My rope is 1/2 inch thick