Samer at http://www.freewaregenius.com/ wrote the following:

HOW TO INSERT KEYFRAME OBJECTS IN A VIDEO FILE

This will enable Hulu videos and the like to be able
to fast forward etc. when keyframe objects are not
included in the video file.

The reason FLV videos downloaded from Hulu do 
not fast forward is because they lack keyframe objects 
(little markers interspersed across a video that enable 
the media player to navigate back and forth). Hulu 
videos probably do not contain these by design, to make 
it harder for people view videos downloaded from Hulu. 
This is easily fixed, however.

Download FLVMDI, a tool that can inject keyframe objects 
into FLVs.

Go to the FLVMDI website. Scroll down to the “Downloads” 
section (towards the bottom of the screen) and download 
the latest versions of FLVMDI and FLVMDIGUI (at the time 
of this writing these were versions 2.95 and 1.05, respectively).  
Extract both FLVMDI and FLVMDIGUI, then move flvmdi.exe to the 
FLVMDIGUI folder. Run FLVMDIGUI.exe and process your FLV. Once 
you launch this program the dialog is self-explanatory:

For the input file, browse to your downloaded FLV video file. 
Note: to process a number of FLV files in batch use”All files 
in a folder” tab. This will let you point to a folder rather 
than a single file and process all the FLV files inside. For 
Output, you can use the same name as the original, which 
auto-populates. Really no need to create a different output 
file name. Check the “Include ’keyframes’ object” box That’s 
it. You can now click the “Run FLVMDI” button on the bottom 
right to process. Processing should conclude in a few seconds. 

You’re done. Your downloaded file should now be fully playable 
on any media player that plays FLV files.

http://www.buraks.com/flvmdi/