Jaycut is a flash-based online video editing tool. It's surprisingly capable, what I'm wondering is how they achieve this in Flash/Flex. They allow mixing of video, audio and applying transitions and that seems a lot to do in AS3. I wondered if perhaps the mixing is done on the server when you click "preview video", but even then it still seems very quick.
Any thoughts? AS3 developers, how would you approach this?
EDIT: MovieMasher does similar things, and seems to suggest they do it all client-side.