That is the "stabilizer bar" a unique piece of mechanical stabilization. It is somewhat rigid in space, because it has weights and is stiffened centrifugally. Since cyclic pitch controls pass thru it, via a linkage, it damps pilot inputs, and also rotor disturbences. In order for the cyclic to get to the rotor blades, the stabilizer bar must be rotated a bit. There are dampers to keep the bar from oscillating while it does its business.
The bar creates a reference for the rotor, so that when a gust disturbs the rotor, the attempt to teeter produces a stabilizing cyclic input to correct the rotor motion. Only pilot control inputs will allow the rotor to teeter.
Some Bell teetering rotors have no bar, and these generally are a bit less stable (but not badly so, frankly). Since the stabilizer bar slows down pilot inputs, it was replaced by electronic SCAS on those models where maneuvering is a virtue. the typical teetering rotor is quite marginal in quick maneuvering, as it is.