It looks like a
servo, it acts like a servo, but it's more than just a servo! We got a factory
to custom-make these classic 'standard' sized hobby servos with a twist - the
feedback (potentiometer wiper) line is brought out to a fourth white wire. You
can read this wire with an analog input such as those on an Arduino, to get the
servo's position. That information can be used in robotics to improve stability
or even allow 'recording' of servo motion.