looks like it is designed to articulate. when accelerating the free bolt will push down against the spring locking the traction bar and limiting twist, any other time it will be free to move. the reason for this is to stop rear steer as the wheels ride bumps and dips in the road with the traction bar pulling at the axle. rear steer can be exaggerated with traction bars at different ride heights as to whether the fuel tank is full or empty, trunk full of stuff etc.
you could put a rubber sleeve on that bolt to stop it clacking against the spring when you hit the throttle.
