300tdi injection pump arm. What is it?

What is the purpose of the second arm on the pump that is moved with the actual throttle arm?

Mine doesn't seem to spring back under the spring load after the throttle lever is pulled

I thought it was for a hand throttle

I've wondered that myself, I think it is only present on 300 FIPs that have EGR and throttle position potentiometers. It won't be hand throttle as it doesn't control RPM, I think it either changes the injection timing or potentially some sort of cold start device but mine doesn't seem to make any obvious difference when you move it.