Transparent FLV video in flash presentation

I see this trick at few websites:
a) one part of flash presentation is normal vector flash objects
b) second part is external flash video (FLV) file added to flash movie, which have one color background - this color in flash animation is transparent, at this video for example is human, car…

How it’s made?

[the same in deutsch: ]

[if loaded click after right side of page content at cup of coffe,
and next if presentation is loaded click “zamknij” (close)
and move quickly mouse coursor to see the some animations]