At the beginning of the Cairn Reactor level, you have to run past two laser beams, kill a Reborn (Normal difficulty and above, I think) and use Force Speed to run up the tunnel before the laser activates again. After blowing up some laser projectors (and blowing holes in the wall) and a few more quick runs, you'll come to a hub with a four lasers attached to a central core. There's a switch on the core. Once you've flipped it, run back to the spot where you blew up the lasers and look up. You may encounter a Reborn at that point, depending on the difficulty level you've set.