Formatt's 4x5.65" Reverse Graduated Neutral Density 0.9 Vertical Filter is unlike regular ND grads where the graduation is from the top of the filter, getting lighter towards the middle. This filter offers the transition from dark to light from the middle of the filter upwards.
Normal grads are designed to reduce the light when the sun is high in the sky, while reverse grads provide control where the light source is lower in the sky. This is a benefit when shooting bright horizons to enable more control over the exposure and balance the foreground image.
In particular this filter is very useful for shooting around sunrise and sunset where the main light source is at or near the horizon.