The shutter speed on digital video is analogous to the shutter angle on film motion picture cameras (Google shutter angle for a better understanding). The shutter speed of 2x the frame rate simulates a 180 degree shutter angle which is the normal in film motion pictures.
The shutter speed determines how long the sensor is exposed to the light for each frame and thus directly relates to the amount of motion blur. In moving pictures (film or video) you are showing your brain multiple still images per second (frame rate). Typically this is 24 frames or 30 frames. Your brain receives these images and perceives differences in these images as motion. For the brain to "see" these images as normal motion there needs to be a degree of motion blur between subsequent images of moving objects. Setting your shutter speed to twice the frame rate provides the brain with a comfortable amount of motion blur so the "moving pictures" appear to be realistic.
If you "up" the shutter speed to a faster shutter, you reduce the motion blur and your brain starts seeing the video as "choppy" because moving objects seem to jump between each frame rather than move smoothly from frame to frame. The faster the shutter speed, the greater the effect. This can be used on purpose. For instance the first 20 minutes of Saving Private Ryan was shot this way to give the audience a jarring, hyper-realistic experience.
So, if you want to obey the 2x shutter speed rule and you have a camera with a fixed aperture (Mavic), then the only way to get a properly exposed image is to use an ND filter to reduce the light getting to the sensor.
I hope this helps. I have tried to distill some complex stuff down. For those who want to learn more, leave the drone forums and search online for digital cinematography or videography resources.
