Article relates to
RadSplitter for ASP.NET AJAX
You can find a working project which demonstrates the described approach in the attached archive file. NOTE: In the Q3 2009 Beta release a new property was introduced which offers the above explained functionality out of the box. The property is called DockOnOpen and it belongs to the RadSlidingPane.
The official Q3 2009 release offrs a sample online demo which shows the DockOnOpen property effect and which is available below: http://demos.telerik.com/aspnet-ajax/splitter/examples/sp_dockonopen/defaultcs.aspx