First of all, a frame within a frame occurs when you use a visual element in the image to frame the primary subject. The purpose of using a frame within a frame is to help move the viewer’s eye toward that subject.
The beauty of incorporating a frame within the frame is that it can be virtually anything you want – a doorway or window, a cave or tunnel, or even foliage, clouds, or areas of light and shadow.
In the image above, note how the shafts of light coming down from above frame the primary subject – the man standing atop the rock. Note as well that in this case the frame doesn’t necessarily obscure anything, but works more to direct the eye where you want it to go… More at photographytalk.