Basically the yellow tool signifies that between one position and the next is a large difference in axis values, e.g. when you go through a singularity and A4 flips.
However, in the current public PRC version, PRC doesn't yet track the axis values properly so sometimes you get strange readings. The new PRC version that can currently be tested by members simulates and tracks all positions, so that you can also spot when e.g. the robot "winds itself up".
Johannes @ Robots in Architecture