|Spoiler Alert: This page contains spoilers. Read at your own risk.|
Normally, a CAD facilitates the invocation of magic by providing the Activation Sequence, which is then absorbed into the Magician's Magic Calculation Area, where it is translated into a Magic Sequence.
Thus the spell building process is:
Caster - Psion Signals inserted into CAD - Electrical Signals within CAD to activate the Activation Sequence - Activation Sequence sent back into caster as electrical signals via nervous system into the Magic Calculation Area - Construction of the Magic Sequence - Spell Output.
However, with the Activation Sequence mentally carved into their brains, Yotsuba Family members can directly recall the Activation Sequence from memory without the aid of a CAD. The Magician is able to directly start the process of creating Magic Sequences in the brain. This essentially eliminates the time spent converting Psion signals into electric signals, processing of the electric signals through the CAD, and then sending of those signals into the Magician's Magic Calculation Area. This bypasses all the steps required before the construction of the Magic Sequence and allows for casting times that are at least as fast as the casting times with even the most specialized (and the fastest) CADs.
There are several limits, however. First of all, members can only Flash Cast spells that have been carved into their brains (this process is implied to be both exhausting and painful). Second, the limit of Flash Casting is in the title itself: Single Systematic Magic Casting Technique. Thus, Flash Casting is only possible for spells that use a single system of magic; overly complex spells that involve multiple systems cannot be used with Flash Casting. Tatsuya stated to Mizuki in Volume 2, Chapter 8 that his personal limit with Flash Casting (although he doesn't call it by name) is spells up to five processes. It is unclear if this limit is relative or absolute among the Yotsuba Family.
It is probable (but unverified), that the reason why the Yotsuba are able to use the Flast Cast while many others even after knowing about are unable to, may be because the Yotsuba being part of the 4th Research Institute's (whose faculty or area of research being related to the mind) and the possibility that their mental landscape has been altered to allow use of Flash Cast (reference Visitor Chapter (III), Check related Quote here)
The Yotsuba Family have a secret method of directly and forcibly injecting information into a person's brain.
- Tatsuya takes Flash Casting to a whole new level. Since his natural subconscious Magic Calculation Area is completely dominated by his two natural abilities, Decomposition and Regrowth, Tatsuya relies on his artificial Magic Calculation Area that was implanted in his mind for Flash Casting. While his artificial Magic Calculation Area is highly inferior to natural Magic Calculation Areas, his artificial Magic Calculation Area lies within his conscious mind, as opposed to the subconscious mind. Tatsuya can see not just the Activation Sequence of a spell but also the Magic Sequence itself. This lets him bypass even the last step before the spell takes place: the construction of the Magic Sequence in the mind. Instead of building the Magic Sequence, he can store an image of the Magic Sequence itself in his mind and directly cast the spell - the only true instantaneous spell casting. It is also implied that unlike any other Yotsuba Family member, Tatsuya is able to Flash Cast any magic spell with five processes or less even without the brainwashing of spells into his minds. For battle purposes, this is incredibly formidable as most combat spells involve fewer than five processes.
Note: They are not the users of the Flash Cast Technique but the technique is similar.
- USNA Magicians Possessed by Parasites
- Magic that hasn't lost any of its casting speed without the use of a CAD is known as "spiritualization" by the STARS, showing that at least some magicians in the USNA (aside from the ones who were possessed by the Parasites) are able to do this.