My cheap RC mini-helicopter:

The single-axis gyroscope inside the helicopter:

It is used to detect if the helicopter rotates left or right unintentionally while the main blade is rotating. The MCU inside the helicopter would try to control another motor to rotate another blade to counter the rotation if necessary based on the gyroscope reading.
Here is my video showing the balancing control of the helicopter:
Furthermore, there are some articles worth to read: