[Szayel has gotten very good at that same skill thanks to having to work with his hated enemy for so long]

Oh I am sure my dear colleague would beg to differ. And... in a way. It's more like... the very power of your own soul. Or, rather, the pressure it exerts on the spiritual particles around it. A translation of "reiatsu" is, indeed, "spiritual pressure". We Arrancar, as well as Shinigami and Quincy, use this sixth sense of ours to track and detect others. But enough about that... what is this "Saint Graph" you speak of?