When it comes to filming, using a fast shutter speed is generally a bad idea. For most applications, you'll probably want to keep your shutter speed somewhere around 1/60 of a second, maybe even slower, depending of course on what frame rate your shooting at. The reason for this is because it allows for more smoother and cinematic looking footage which isn't choppy or harsh looking. Faster shutters speeds generally can be a little jarring to look at. The problem with this is that to compensate for this slower shutter speed you may need to stop your lens down. This, in turn, prevents you from getting that shallow depth of field, especially when filming outdoors.
Imran Mirza, a professional photographer and YouTuber, demonstrates how the latest variable filter from NiSi can allow you to shoot with wide apertures during the day. Filming with the Canon EF 85mm f/1.2L II, Mirza was able to shoot wide open comfortably without the footage looking overexposed. Although there are many other ND filters currently available on the market, there are a few features the NiSi variant offers that make it much more compelling. For one, the transition between the stops on the filter is incredibly smooth, and it's almost like having a de-clicked aperture ring for the exposure. The markings on the filter also help you clearly see how much of the effect is being applied and even at maximum application, there's no sign of the horrible "X effect" in the footage.
Check out the full video to see the how the filter performs.