It's this thing here.It's not on the download page, as it mentions, as this is not yet at beta stage.
It works by controlling the time in FSX, so that you get a steady FPS, even if it takes three times that long to render:) I finally gave in and installed it (I never install alphas...) and it seems to work well. It switches FSX to window mode and resizes it to match the video size. My first test recording gave a nice smooth 3 minute video, using three recorded aircraft plus a POV slew. However partway through I got a G-force black-out, then a red-out.
There are some gotchas, such as the fact that you can only render a recorded flight, you can't fly in real time. And no sound, of course, because of the variable time frame. They do suggest that you record the sound separately by replaying the flight as normal.