Usage of FLVTOOL
You can download flvtool from official website
FLVTool set meta information that is used by flash player to properly stream flv videos.
If FLVTool proper set meta information, then original flv file size increase about 1kb.
It is useful when you implement partial streaming like youtube does.
In partial streaming user can stream video from any location. from start, middle or end of video.
Also if meta information not set for flv videos most of desktop flash players like http://download.cnet.com/FLV-Player/3000-2139_4-10505954.html showing Shows bitrate as 0kb/s.