ORIGINAL: Mr. Longbeard
I think they do slide down the arrow shaft...
I think you are confusing the grashoppers with the zwicky scropio's. The Muzzy site actually says this:
Muzzy Grasshopper Rings
Great for stump shooting and
small game. Won’t skip or
bury under grass. Springloaded
rings can be added
behind any screw-in field point
or any style tip. Ring s are 20
grains each.
Below is a link that shows a picture of the grasshopper
http://www.eders.com/Muzzy-Grasshopper-p571c108-path0.html
Here is a link to a picture of the Zwickey Scropios
http://www.3riversarchery.com/Product.asp?c=10&s=43&p=86&i=4205X
If you look at the pictures of both, you will see that the grasshopper has the head screw tight against them while the scropio slides onto the arrow itself.
Hope this helps.