Telerik RadAjaxManager still postbacks

Question

I've the following code in my page:

<telerik:RadAjaxManager ID="ajax_manager" runat="server" 
    DefaultLoadingPanelID="ajax_panel">
    <AjaxSettings>
        <telerik:AjaxSetting AjaxControlID="mygrid">
            <UpdatedControls>
                <telerik:AjaxUpdatedControl ControlID="mygrid" />
            </UpdatedControls>
        </telerik:AjaxSetting>            
    </AjaxSettings>
</telerik:RadAjaxManager>

The "mygrid" above is a RadGrid, with grouping and paging. The purpose is to expand the groups or change the page "without" postbacks (I think that should be the objective of the RadAjaxManager, anyway).

But after all, the components still postbacks... =(

Some hint of what could it be?

Regards!