The rapid seekers happen when you "curse" the frozen springs on battle3 (or any other level) by blowing up TNT next to it after its warmed up, then jumping on them, and pressing shoot. As long as you're touching the messed up springs, you'll have hyper-fast-fire that works with any weapon. Now that I've played around with it a bit more, I realize the path of the spring always get pushed in the opposite direction of the TNT, so by placing the TNT well you can force the springs to do useful things. For example, if you have some spings next to each other, you can make each one push you into the other one, so you'll continuously shoot rapid-fire in both directions. Or if you place the TNT directly over the spring it'll become a trap, and will hold someone in place. It's possible to escape by pressing jump and sideways, but it would make someone an easy target for a while.

Edit: Forgot to mention, if the TNT is placed directly under the spring (which is done most easily by placing an event 58 generator on the next lower tile) it will launch you upwards, but with a pathetically small force. Maybe 1 or 2 tiles. I suppose this could be used in a level to make a spring which is weaker than a normal red one, altough I'm not sure when this would be useful.