From the manual :
The autoplay or preload mode. This can be:
No: nothing is done until the video is requested to be played.
Preload: on startup the video will start downloading the video data, but will not start playing it yet. This can allow video playback to start more quickly when requested. Some platforms (e.g. mobile devices on cellular data connections) may ignore this.
Yes: on startup the video will start downloading the video data, and also start playing it as soon as it determines the progress and transfer rate are sufficient to play through to the end without stalling for buffering. Some mobile platforms will not start playing until the first touch event - see Compatibility for more information.