I've always wondered how iMovie plays back without any kind of rendering indicator. Either the things you can apply just aren't big enough that it needs rendering, or it renders in the background like FCPX, but without any indicator. I assume the former. I know that some of the older versions had to render. I don't actually use iMovie though, can't quite remember how limited it is...my biggest problem with it is that the timeline isn't accurate, it's horrible to use.