In the encyclopedia illustration showing the Pioneer space probe paths, what was the significance of the lines with the comment “equal pull?”

This was a depiction of the gravitational influence as well as some other types of energies that were acting on these spacecraft and affecting their trajectories, to some degree, as well as the interplay of energy from the types of emissions of the heavenly bodies in question. So there was an illustration put together to represent the fine-tuning of the space probes in planning their orbital mechanics with the forces brought into play from liftoff through circling the Earth and being slung out into a track that would take them out of the solar system ultimately and into deep space. The considerations here were part of secret plans that have still not been revealed.