Bob Harrington wrote:

In effect, there is a narrow slit that moves across the film exposing it
as it goes. *While the shutter speed may be set to only 1000th of second,
it could well take ten times that long for the slit to cover the full
frame of film. *If something in the scene like a propeller is moving very
quickly, it can move visibly during that time, resulting in the apparent
warping of the propeller.

That is interesting. Thank you!