Hi Kishor,
I think the confusion is that you hold down alt over the button/option in Blender before you click it. The mouse is context sensitive so it will only tumble the view when it is over the 3d window, when it is over other areas of the UI it will do different things.
You can use this ALT click tip for many many many different things in Blender – it’s a global feature.
However, if you are using any version of Blender other than 2.91 there is a shortcut for changing the rotation mode of the selected bones in Pose Mode.
That would be CTRL R
NB – This shortcut does not work in 2.91 because it was removed by accident.
Hope that helps.
