Setting this property to true will make the WindowSplitter go in LiveMove mode on MacOS Carbon systems.
Flickering will only depend on how good other controls you are using are at drawing them self without flicker, and the operating system. MacOS X for example does most things double buffered which reduces flicker, but control drawing on MacOS Classic is not double buffered so it might not make as much sense to enable live movement on MacOS Classic.
This property has no effect on Cocoa. On Cocoa the move is always live.