As is clear (I hope) from the above top graphic, if you apply the 180 rule to exposure when doing video, the frame is exposed for half of it’s lifetime.
Say you have chosen 25p (25 frames per second) for your video recording. Using the 180 rule you’ll set your exposure to 1/50th and you’re capturing half of the action that takes place during your frames lifetime.
In other words, Choosing 25 frames per second means you’re sending 1 frame to the viewer every 1/25th second. If you expose for 1/50th second, you’re exposing only half the lifetime for each frame.
So let’s say a car travels 40km / hour. That’s 11 meters per second and half a meter per 1/25th second. So if you expose for 1/50th second, every 1/25th second, your actually recording a car 1/4 of a meter for every frame and skipping the other 1/4 meter. So you see, now you are recording video with gaps. (see 2nd graphic).
Ok, So now let’s look at what happens if you run 25p for your video and expose for say 1/100th of a second. That means that you’re exposing 1/4 of the time and leaving 3/4 of the frame time unused. If a car travels 50cm in 1/25th of a second, then 1/100th of a second will only capture 12.5cm.
One frame will show 12.5cm of movement (blurred), then the next frame will show another 12.5cm and so on, with the beginning of each frame starting 37.5 cm from the end of the last shown distance (see 3rd graphic)
You see?
In all cases you are missing part of the footage :)
In the case of 1/100th of a second exposure you are missing 3/4 of the action that happened during the one frame time (1/25th sec), in which case your eye could start to see jerky footage.
The 180 rule is a rule of thumb that makes it kind of acceptable to watch, with your brain making up for the missing information.
Why can’t we have full frame exposure?