The rotary steerable drilling tool and system described herein combines both point-the-bit and push-the-bit techniques to actively change the direction of the borehole trajectory. In this system, the deflection of the drill bit is limited to a single degree of freedom relative to a coordinate system that is fixed to and rotates with the rotary steerable drilling tool, resulting in a simplified attachment of the bit assembly and bias unit mechanics. Further, steering of the well is accomplished by dynamically controlling the spatial phase and amplitude of the coherent symmetrical bidirectional reciprocating deflections of the drill bit relative to a fixed terrestrial datum as the tool is rotating, simultaneously pointing and pushing the bit. The amplitude and force of the bit deflections can be variably controlled during steering operations to dynamically adjust the instantaneous build rate as needed. When steering is not active, the bit can be mechanically locked into the neutral position.