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Ok, here is the deal....a friend of mine wants me to add a trail of fire effect (like in Back to the Future) on a video clip of him on his snowboard. I have a wall of flame clip already keyed out...how can I add it to the clip and have it sort of sweep across the screen behind the snowboard, if that's making any sense? I tried dragging the side over until it's off the screen and set a key frame, then have it follow along behind the snowboard in the clip, growing out to full length by the time the snowboard is out of frame...but it didn't seem to work the way I had expected it to. What am I doing wrong?

This should be a very simple effect to pull off, but I just can't figure out what I'm missing.