Once the animation has started, the currently selected item is already the new item. Therefore in the OnClientItemClicked event handler the args.get_item() will return the new item, not the one that hiding. Also the SlideShow animations - Fade, Pulse and CrossFade, cannot be cancelled due to other restrictions in the functionality. The stop, start, pause and resume methods of the RadRotator will work with the other animations of the RadRotator, which are involved with sliding the next item within the viewport of the rotator.
Hope this answers your questions.
the Telerik team