touchscreen - What exactly does a capacitive touch screen need to register input, and how can that signal be produced?

There are active styli out there, and they take 3A or 4A batteries and don't require any wireless connection. How do those emulate human touch?

EDIT: I am specifically interested in iPad touch screens. After some research, I have realized that there are many types of capacitive sensor tech, but most people seem to be in agreement that Apple uses mutual capacitive projected capacitance sensor arrays. These are set up with a grid of capacitors. A voltage is applied to one axis, and the other axis reports change in voltage when the local electric field of a finger alters mutual capacitance of the capacitors directly underneath it.

EDIT 2: what I really want is something with a fine tip like a ballpoint pen to project about a 5mm electric field the same way that a human finger does. There are styli on the market that try to work with a fine tip, but they all fail and I do not want one. I want to make my own because DIY projects are way more fun than buying an overpriced failure of a product.

Answer

How do those emulate human touch?

Capacitive touch screens don't work from the supposed "aura" that some folk believe emanates from humans - they'll work with dogs, cats, medium sized rodents, pieces of metal and probably wet wood. Maybe they'll also work with high dielectric plastics too.

Capacitance is not given by gods to humans but is firmly routed in the physics of objects and the "styli" or pen has to be held in the hand and therefore a significant capacitive connection to the body is made (whether wireless or otherwise).

I suspect that wireless styli that use batteries may work by sensing the capacitive sensor's AC field and re-injecting some form of antiphase signal - the disruption is likely to be enough to "register" a keypress.