I've always wondered about that piece, too. Rotating it closes a flap in the intake manifold. From reading a few threads on here some time ago, I believe diametricalbenz is correct, that the flap is designed to close if somehow one managed to start the engine rotating in the reverse direction (where the intake would become exhaust), allowing it to be shut down.

Mine doesn't have a spring like the one in the video, in fact I've never seen any other W115 diesel that had one like that. Though I think it's odd that it would be free to move back & forth.

I wonder about that linkage with the flexible rubber boot in the picture - on my car, and others that I've seen, this is a straight metal rod. Did you observe its action when you move the throttle linkage? If it's flexing....that could cause your problem when you put your foot to the floor.

Otherwise, I'd think a loss of power when you hit the throttle would be caused by a momentary fuel starvation.

Putting the pedal to the metal on my car is very underwhelming....it doesn't speed up much at all until the revs get into the very narrow torque curve and then there's a sudden surge of power. I think that's normal, but I'm not sure...I drove a W123 240D once and the acceleration felt more linear.

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1976 240D "Katja"