Coiled for action means prepared or ready for action or ready to act or fight back. I think this is akin to how coil springs work. When you compress a coil spring, it becomes very tense with all that stored energy that you impart to it. And the moment you let go of it, it pops back to its uncompressed state releasing all this energy. That's one way to think about it.
Another way to interpret the meaning of this expression has to do with snakes. When snakes are frightened, they coil up their body into a spring-like shape, thereby demonstrating that they're ready to fight back if they're attacked. I think that's where this expression actually comes from.