Step 1: History
In an experiment, I connected a loose HD to a computer power supply, and felt the resistance to its change of position. I figured a HD in the horizontal and one in the vertical plane (X- and Y- axis) would dampen almost all unwanted shaking.
The 2 Gyro stabilizer model is very successful, but practical use of the device is limited, due to bulk and weight.
A single Gyro has its theoretical limits. A strong Gyro in the front / back plane (Z - axis) could dampen shaking, but would not correct rotation. But if rotation around the optical axis is only a minor component of motion blur... , the advantage is less weight and power requirement.