The MG5921HV is part of MacGregor's new range of high voltage digital servos. This high voltage, high torque servo has been designed for use with large scale & aerobatic aircraft.
Battery chemistries such as Li-Po, Li-Ion, and Li-Fe have become very popular within the RC community. They are the standard for powering electric motors and have also gained popularity as receiver battery packs. The issue is the output voltage of these batteries is higher then the previous standard 4 - and 5-cell NiCd and NiMH packs, 4.8V and 6V respectively. The higher voltage is not an issue with the receivers but can be for the servos. Many servos will not operate at these higher voltages for very long before burning out the motors or electronics.
To get around this issue modellers have had to use a regulator to drop the voltage to an acceptable operating range for their servos, usually around 5 to 6 volts. The addition of a regulator adds a component that, should it fail, can cause the loss of a model.
MacGregor's HV servos are designed with motors and electronics that operate off the direct voltage of a 2-cell Li-Po, Li-Ion, or Li-Fe receiver battery pack. This eliminates the need for a regulator and lowers the chance for a power failure that can cause a catastrophic failure of your model. Also note that the HV servo specs are shown at the higher voltage (8.4V 2S LiPo) but will still operate on a standard 5-cell 6V NiCd, Ni MH battery with a reduction in performance of approximately 10% in speed and torque.