I guess you can have an "overcast" day where the sky is still killer bright, relative to the land, so a graduated ND filter will take the heat out of the sky, allowing you to then balance the ambient light to manageable levels. Here's an example where the sky was overcast, yet to hold the detail in the sky I had to severely underexpose the foreground. I did not have an NDG filter.
Here's how it looks when adjusted in software simply to give an exposure boost to approximately get the exposure OK for the foreground. When I did this in camera the sky was completely blown.
Now I didn't need flash for this shot, but a graduated ND would have evened out the ambient light to make the exposure for this picture far easier to manage. the wiggly horizon is a bit of a killer for using an NDG but hopefully the example is made, nonetheless.
This isn't a great scene into which to plant a model to be lit by fill flash, but assuming that is what you wanted to do, with the ambient light better balanced between land and sky, you are then free to use the flash as you like to balance the lighting of your subject with the background. If you didn't have an NDG for this shot, and lit a model with flash, you'd end up with bright sky, miserably dim middle ground, and a well lit model.
So at the end of all that, I don't think there is any special technique or trickery in play. It is just a question of first sorting out the ambient conditions and then sorting out your subject and his/her/its lighting.
Then again, perhaps I have completely the wrong end of the stick. You did say initially you were using a 1.3X crop body, which I took to mean a 1D something or other, but now you say it is a 40D, which is a 1.6X cropper and probably puts you a bit further from your subject then I first estimated, so comfortably 2m rather than anything less. You've also sometimes said ND Grad and other times just ND, so I'm not really clear on what filter you are talking about. If it is a grad and goes on the lens then you're going to have a rather odd gradation of the flash light, with the top of the head lit less well than the chin/neck/chest etc.. It all seems very odd to me. If you were shooting from above your subject then I could understand why you might need a gradient on the flash illumination, so that the head/face, which is nearest the flash, is dimmed, and the body, which is further away, gets the full flash with the clear glass. But really I'm just guessing wildly here. A sample picture showing the problem you have at the moment would help, as would some links to these places where you have seen the technique mentioned.