The Chaos, or Natural, magician believes that all things are inherently magical. Magic, to him, is dynamism, potential, and possibility. All things, even those which are completely inexplicable in logical terms, are possible. What is required is making that which is merely possible probable.
Chaos magic is a sort of magical understanding of what we now refer to as chaos mechanics. In a complex system, a small, even unidentifiable, change can produce unforeseeable and spectacular consequences. The Chaos magician can control this apparent instability and bend reality to his will. In a sense, a Chaos magician finds the loose threads of reality, and simply pulls.
It is important to realize that the actual causative magic the Chaos magician employs is very minor. Most of his work goes into understanding the system which is to be manipulated. Aside from the small triggering magic, everything which ensues from the magician's spell is completely explicable in mundane, scientific terms. It's often therefore seen as the most unmagical of spheres, and its magicians are often disparagingly referred to as "statisticians".
Sometimes Chaos magic has no direct effect; its only purpose is to manipulate the probability of certain outcomes. While it is the most "reality-bound" of all the spheres, it is also because of that the most subtle.
The actual causative magic the Chaos magician employs is very minor. Most of his work goes into understanding the system which is to be manipulated. Therefore his work rarely requires much effort in the form of Stamina expenditure, but it is almost always difficult, and usually utilizes Intelligence or Perception.
There are no special rules related to Chaos magic, but many of the spells directly affect the way the game calculates the probability of an action's success; for more information on this, you should check out the page on dice.