The point is, that the ring around the penis will move up and down the penis,
as the knee is moved e.g. by walking.
The piston connecting the belt to the knee strap is covered with a flexible
plastic sleeve, in order to protect the wearer from the oil making the piston
work smoothly.
(I'm sorry for the lack of picture quality...)
You can download a 'building-manual' (in *.pdf format) here: 
And I've had the masturbator belt out for a little shopping, click here to
see the video 
-and now I've also had it for a walk in the forest, click to see...