There is no spring in the upper pivot. It's a standard drop parallelogram rear der. It's notable for its beautiful industrial design and excellent shifting.
The pulley cage is adjustable--loosening a bolt allows the whole cage to slide up and down to adjust for different size freewheels. This feature works very well, although you must take great care when retightening the bold because it has a tendency to split in half--it takes very little torque to break it.
Another feature of this mech. is that its pivot pins are held in by circlips, making it a breeze to replace parts.
I used these for twenty or more years on several bikes, sometimes with pretty wide gear ranges. It's a great derailleur.
Bob Schenker
Oakland, CA