Gyroscopic Motion - I

We observe that precesional angular velocity is inversely proportional to spin angular velocity. In case, the disk was not spinning, it would not have been possible to balance it on A. It would have started falling down in the vertical XZ plane.

However, it does not happen here due to spin motion of the disk. If it has to fall in the vertical plane during spinning of the disk, there has to be a torque in the horizontal plane( represented by a vector along negative z-direction).

This is clear from the following figure:

When the disk has fallen by a small angle, the direction of its angular momentum gets changed, though the magnitude remains same. Change in angular momentum is as shown in the following figure.