When the sun is hitting the horizon, it is often over exposed with a standard ND grad as the lightest part is at the bottom of the frame. The reverse neutral density grad features a distinct transition in the center of the filter. This initial transition into the ND quality is similar to the transition zone in the hard edge grad. The ND effect then progressively lightens toward the top of the filter.
When you are using a reverse grad you can control the light exposure by placing the darkest part of the grad over the brightest highlights, usually at the horizon line. The reverse grad is a vital tool if you are looking to take sunrise or sunset shots.