The older Shimano rear derailleurs (pre 7700) use a torsion spring inside the parallelogram. But with the release of the 7700 group they switched to a tension spring which they continued using ever since.
The 7700 was considered a complete redesign of their drivetrain components. I am wondering why they made this specific design decision. What were the shortcomings of the torsion spring that they hoped to overcome using a tension spring?