This parameter is available only in 2-pass variable bitrate mode. It allows the encoder to selectively drop passed frames when it believes they do not contain enough significant information. It is helpful when you want to produce a stream with the lowest possible bitrate, but retain acceptable visual quality. In this case, the encoder will drop more frames in scenes with low motion or no motion at all, and keep them in the fast-motion scenes.
The default setting for ôFrame droppingö is 0 because usually there is no need to drop frames.