Really great utility that allows you to get under the hood and make a number of low level but highly impactful aspects(including aspect ratios) of a film, including playback speed(not fast forward, but in such a way that the move is made longer or shorter), syncing the audio with the video, (un)cropping, inserting tracks, and a few more.
You open the program, then load the movie. You can either watch from the program, export or save it. It doesn't have a million features, but the one's it does have are very useful, especially with video downloaded from the internet where you find a lot of bad initial coding and unusually large media files that bog down after a while.