It's simple to use. You pick an open window
And you get another window (optionally always on top) that displays th chosen windows' live client area:
From there you can interact with the source window just as you would the real one with mouseover effects, button clicks etc But that's a bit boring, and with large windows the contents are all small like, so what you can do is select an area
and be zoomed into that
Then you can resize the window to enlarge or reduce the size of the content
Finally, because it'd be rude not to, you can still interact with the now zoomed and enlarged content
If you prefer to see it in moving action, I whipped up an 'ad' on my first go round last year.
It requires Windows Vista, 7, or 8 and Aero to be enabled and that's about it. It weighs in at about 150K soaking wet and uses a whopping 1.5MB of memory, so if it looks useful, have a go with it.