Because of regulations at the time the film was made, the Master of Suspense, Alfred Hitchcock, was not permitted to show any contact between Bates’ knife and Marion’s skin. In order to still convey what was going on, he chose to juxtapose shots of the killer’s weapon with Leigh’s agonizing facial expression. Ironically, the montage which resulted from this decision proved captivating not because of what we do see, but what we don’t.

Source: Medium