How do high-speed film cameras work?
A normal film camera has a mechanism for stopping the film in the gate every frame while the shutter is open, then when the shutter closes pulling down the next frame, stopping it, exposing it, and so on. If the film isn't perfectly still while the shutter is open the image will be blurred.

To do this with high speed film is impractical, the forces involved in pulling down the next frame in 1/500,000th of a second and bringing it to a dead stop would simply destroy any film you could put in it. So in a high speed camera the film doesn't stop, instead there is a rotating prism, as stated in the article. The prism bends the incoming light as it rotates, and it does so in a way that the image follows the frame of film as it passes through the camera. 

A good article explaining it here, this illustration in particular:



In this illustration the frame of film would be on the right side, moving downwards, following the spot of light.

The reverse of this can be seen in a Steenbeck editing machine that also uses a rotating prism to hold the image on the screen steady while the film moves continuously through the viewer.