I'm pretty sure that they are using the ND filter as a way to effectively cut down the brightness of the sunlight, to overcome shutter synch speed limitations using non-HSS type flash bursts. So, for example, let's use the so-called Sunny 16 rule as an example, and say that the correct exposure is the ISO speed for the shutter speed. So, if we use ISO of 200, then the shutter speed would be set to 1/200 second, at f/16, in sunlight. And that would be the correct, Sunny 16 exposure. But let's say we do not want to use f/16; what to do,using our regular,non-High Speed Sync type flash? Well, add a 3-stop ND filter over the lens, and then we can still shoot at 1/200 second, but can open up the lens three whole f/stop values, from f/16 up a stop, to f/11; the second stop from 11 to f/8; and the third full stop, is to f/5.6, which with a longer lens and typical portrait distance, will give a more-defocused background than f/16 would have given.
The F/stoppers website had a nice,short article on the benefits of using a polarizing ,filter for portraiture, a couple years ago.[/QUOT