http://msdn.microsoft.com/en-us/library/bdea5dt6(v=vs.80).aspx[
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With this functionality you can scroll a part of your window in any direction you want. The part of the screen that becomes invalid has to be explicitely invalidated so that it causes a repaint for that part of the screen only.
Your paint routine should be written as such that it is smart enough to only paint those elements on the screen that have become invalid. This way you can make your screen seem like it is moving without annoying flickering.