[spoil] Think of a psychic as a mutation. It uses parts of the brain that aren't normally used. It usually only lasts a generation. However the demon hunter families have found ways around this (incest).
Magic circuits are not my forte so we shall use the fuyuki wiki:
The pseudo-nerves within a magus' body. What produces one's own Prana and the pipeline that moves the system or to be more precise, the pathways to convert life energy into Prana, the pathways that take in and convert Mana into something usable, and the pathways that connect to the high thaumaturgy formula (program) systems. What connects the ethereal body to the material plane. What magi are ultimately using is Od, converted life-force and Mana. It's not really correct to say that Mana's being used to cast spells. When Mana itself is being drawn upon, it follows the rules laid out by Formalcraft which doesn't seem to operate by the generation rules of circuits.
The number of these are determined at birth. Lineages do what they can to tamper with their blood in order to birth a heir with even just one more circuit. Once opened, these can be turned on and off at one's will. Lineages and magi do not try to increase the circuits of a heir after birth.
Transplanting circuits is possible, but carries the same risk of transplanting a foreign organ. Circuits don't multiply or decline in number. Sure you can transplant them, but it'd be the same as putting in an extra kidney.
The image of the switch differs from practitioner to practitioner. Shirou's image involves a firing hammer, while Rin's is that of a heart being stabbed by a knife. This is related to the very first time the circuits are opened. There are those that open when sexually excited, those that only open when the user inflicts damage on himself, etc.
As the operation of the power source that circuits are necessitates organic activity, it is a common error to think that circuits need the magus' body to operate. There are rare occasions, however, that should the practitioner cease vital operations, the circuits themselves operate independently. To completely ensure that the circuits are completely halted, the brain must be destroyed.
The reason why Od is recovered after being completely used is because the circuits are operating. On the other hand, without circuits, Od can no longer be generated. A magus that pushes his limits by using power beyond him runs the risk of destroying his circuits.
Circuits are stretched throughout the body like nerves and are broken up into core points and bypasses that connect these points. The bypasses that link the cores are like the synapses of the brain and as such if cut or connected hastily, the core will not change. Strictly speaking, this core itself is the circuit. Also, the actual location of circuits is found in the soul rather than in the physical body. On the other hand, the body's ability to actually express these is pretty important, apparently.
Circuits also have properties that reject external Prana. For example, any sort of spell that is used to control the target will be rejected by the circuits (but not always with success).
As the physical body rejects the use of circuits, when active, there will be a certain degree of pain or uncomfortableness. Furthermore, filling the circuits with Prana will wipe out physical sensations. Therefore, filling them has the same meaning as destroying oneself.....according to Tsukihime Lab. Which would make sense as to why Rin's body wasn't able to handle the stress of absorbing 1000 bits of Prana at once. The human physical body wasn't meant to use circuits.
In a single answer lies a thousand questions.