From iPhone Development Wiki

UISplitViewController is used for displaying two view controllers together. It should only be used for the iPad.

Showing the Master Controller in Portrait Mode

Signature @property(assign,nonatomic) BOOL hidesMasterViewInPortrait;
Available in 3.2 —

By default the master (left) controller is hidden in a pop-over in portrait mode. To make it appear in portrait mode too, one needs to set this property to YES.

This is the mechanism used in Preferences.