Other reviewers have said this is difficult to understand (the author seems French) and hard to implement. I find neither point valid.
The program allows you to apply a transparency setting to all windows with a slider on the lower half of the preferences pane.
On the top of the preferences pain you can set a value for the normal transparency of an active window, and you can set a value for the window when it is inactive. (See screengrab of my desktop - Deer Park on top is, as already mentioned by another user, opaque as it does not support this program, neither does Mac Mail or Finder, which is probably a blessing. Beneath Deer Park, Safari is inactive and is only 10% opaque. 10 % is the minimum value you can give to a window's transparency.)
So, if you want your windows opaque then set it 100%, and if you want to see behind it set the inactive value at 10%. Now, all you have to do is click on part of the desktop that is visible to switch to finder, your inactive window will remain, but fade out, and when you want to switch back, click to activate the window and it returns to 100% transparency.
To find the Alpha settings, click the Application name in the top right corner of the screen, so for Sarari it's Safari->Set Alpha Value.... this brings up the preference pane.
Users may also set a fade time value which measures the number of stages in the transition from faded to opaque which can be turned on or off.
3 out of 5. It's useful I guess, but I can see it being awkward for other users. As it is people who also use my computer are thrown by Expose hiding windows and popping up Dashboard when they stray to the corners. Also, the show desktop (f11 by default) and hot corner feature does sort of make transparency redundant for what I would use it for - however, the author says that this was coded just for fun, and that is exactly what it can be used for.