What are RiffTrax files?
If you were fortunate to have watched Mystery Science Theater 3000, then you already get the main idea. In that show, people (and robots, in fact) would sit in a theater and watch a movie, telling jokes all the way through. This is sometimes called "riffing" on a movie. As the viewer, you sit behind these guys and watch the movie but also hear their commentary.
Mystery Science Theater 3000 (aka MST3K) is no longer on the air, but many of the same folks behind the show have started the website RiffTrax.com. They record and sell audio files with this commentary, carefully timed to be played back along with the movie being "riffed". You can then rent or buy the movie any way you want, and simply play this audio file back in sync.
Unfortunately this creates a problem. The audio commentary file needs to be played back in perfect synchronization with your movie. If they are off by even a fraction of a second, the comedic timing of the jokes will not be right.
The RiffTrax folks do a couple things to help you keep in sync. First, they start the audio file with verbal instructions, such as “when I say pause, stop this track and then un-pause it when the Paramount logo fades to black”. This is helpful for getting the sync set initially, but if you pause the movie (bathroom break, etc.), then the sync is lost.
Another bit of assistance built into the audio commentary is known as Disembaudio points (aka sync lines). These are lines of dialog from the movie that are repeated in the audio commentary at the exact same instant. If you are not in sync, this makes it obvious.
Of course, making all this synchronization stuff easier is why we wrote RiffSync. In most cases you can simply start enjoying your movie right away, skipping the “get ready to pause” instructions and ignoring the Disembaudio points. RiffSync will keep the audio file in sync with the movie, and it will stay that way even if you pause the movie or jump to a different location.