This extension offers video and audio combining (combine a video-only stream with an audio-only stream to get a normal playable file) which is done using FFmpeg command-line tool. This feature is optional. You can enable it on the options page. For this feature to work, a native client needs to be installed. If the native client is available, then the extension downloads an extra audio file when a video-only stream is selected. When both files are completely fetched, the extension sends the local paths of these files to the FFmpeg command-line tool to merge them without losing quality.