To allow the first or second panel of the SplitContainerControl to be collapsed when clicking on a splitter, set the CollapsePanel property to SplitCollapsePanel.Panel1 or SplitCollapsePanel.Panel2.
When a panel is collapsed, clicking on a splitter restores the panel to its original size.
The following image shows a SplitContainerControl with the CollapsePanel property set to SplitCollapsePanel.Panel2.

To prevent a panel from being collapsed/restored, you can handle the SplitGroupPanelCollapsing event and set the event's Cancel parameter to true. To respond to collapsing/restoring a panel, handle the SplitGroupPanelCollapsed event.
To collapse/expand the panel in code, use the Collapsed property.