The silhouette rule is a guide for film photography. The camera should be set on the position from where the silhouette of the actor changes the most in the action. For example, when the action is punching, then the camera should take the action from the side to catch the change of the actor's silhouette or the stretch of his arm.
This rule should be applied also to a car and a motorbike. When the action is the jump of the motorbike and the director wants to show just the height, then the camera should be fixed on the front. If the camera is set on the side and follows the action, the silhouette does not change much and the aim of the scene is unclear.
Moreover, Akira Kurosawa used to take the scene of the driving a horse by telescope from the side to emphasize the speed with the fling away landscape – not of the horse, but the silhouette of the landscape is important as the speed. On the other hand, Steven Spielberg, who has studied much of Kurosawa, makes however the most of wide-angle from the front to astonish the audience with the jumping up figure. Both techniques are based on this silhouette rule.