VBV Buffer size - the real meaning

This sets how large (in seconds) to set the Video buffer. The compression will be done so that the average bitrate over the VBV buffer is the requested bitrate.

This method of handling variable bitrates for streaming is not necessarily optimal, since the maximum size of the VBV buffer is smaller than most hard-to-compress sections of video, and there is a playback delay proportional to the size of the VBV. This also leads to wasting space in easy-to-compress areas, as the video size will be increased to fill the buffer.