In order to use the Live Preview feature of the Ribbon Gallery, a developer should use the ActivatePreview and DeactivatePreview events. When a Gallery Item is hovered, the ActivatePreview event is raised. When the mouse leaves the item, DeactivatePreview event is raised. A developer can handle these events on both RadGalleryItem control and on RadGallery control.
For more information about the ribbon gallery, take a look at the Ribbon Gallery topic.
For a full list of the exposed by the RadRibbonView events, take a look at the Events - Overview topic.