For reverse GND, the darkest part of the filter is placed at the transition.
This is especially useful in sunrise / sunset, where the brightest portion of the scene is the sun at / near the horizon. The reverse GND darkens at that spot instead.
* Left - reverse grad. darkest in the middle
* right - their usual GND. darkest from top
Note that some say their hard stop GNDs have a softer transition than some other brands.
I have no other filter brands experience, can't comment
Here is an example of a Vari ND stacked with a reverse grad
http://farm4.static.flickr.com/3202/3108165947_c2928aa753_o.jpg
Ryan