Why is it called fishplate?

Because it is stronger to use a plate shaped like a fish mouth, <> than a square piece like this []. The later adds a stress point directly across what you are reinforcing, especially if you are welding it, which is usually how you add a reinforcement piece like that. <img src="/forums/images/graemlins/smile.gif" alt="" />

If you are adding boulder bars, or rocker skids, you should use plates like that to attach the supports to the frame. The frame is in constant tension/compression, and adding a vertical stress riser, like a weld, is not a good thing. For your trailing arm, you have the same situation, except the point of the highest stress is directly across the arm (like where it breaks).

Michael