The easiest way to do it is to play the video in VLC player and click record button to start recording when you want, then press the button again to stop recording.
The recorded clip is by default saved in your Documents folder, automatically named as vlc-record-YYYY-MM-DD-TIME-NAME-.XXX
is the date when recording is made, for instance today would be 2010-12-07
is the time when recording started
is the name of the original video
is the extension, telling the file type
Recordings are by default saved in same format as original, for instance recording from an AVI creates an .avi file and so on
VLC free download: VideoLAN - VLC media player - Open Source Multimedia Framework and Player