I saw this several times in different movies, but I don't understand what is this supposed to mean. The main idea is when the scene is about to change, for one second or so we still see the picture from the old scene, but the sound is already coming from from the upcoming scene. Why do they do this? What's the purpose of this technique?
In short one have J-Cut and L-Cut
- J-Cut aka audio advance, audio starts before video.
- L-Cut aka video advance, video starts before audio.
It can advance the story in itself by moving the viewer ahead. It also can give a smoother transition, or many other effects.
Find the two examples in the article by Chrisopher to be nice examples.
- Waltz for One from INTELLECTUAL PROPAGANDA on Vimeo.
- Handmade Portraits: Liberty Vintage Motorcycles from Etsy on Vimeo.
For example in second video, the packing of the cigarette packet is smoothed out by the J-Cut, rather nicely.
It's a throwback to the times of movie film projectors. A strip of cellulose would be pulled across a bright light, and then projected onto the screen.
The sound would be from a "sound track", parallel to the image, on the side of the cellulose strip, which would go across a (much) smaller beam of light into a light detector, which would modulate an electric current which would become the sound.
Since the projection beam would be on one part of the projector, and the sound converter on another part, they would be separated in time -- but this separation would be easily compensated for when the film was processed.
The problem comes when two bits of film would be spliced together -- at one point, the projection beam would be shining through one piece while the sound beam would be going through the other. Therefore, the audio would advance to the next scene before the video.
While there is no reason to do so today, some directors like the effect and so do it anyway. Kind of like shooting in black and white or silently to create an atypical result.