I'm not exactly sure on the rules of magic and psion interference. This confusion is kickstarted by the attack on bus on the way to the 2095 NSC and the discussion about flying-type magic. I'm using this discussion for the basics of magic activation.
In the 2095 NSC bus incident, multiple students simultaneously activate magic on the same target. According to the Cast Jamming page, this is an example of pseudo-cast jamming, where the psion noise prevents a magician from communicating with their CAD, and also reduces the effectiveness of already finished magic sequences in the area (??).
As the magic sequence is written to the Eidos, Tatsuya cancels the magic sequences with Gram Dispersion, decomposing the magic sequences into meaningless psion noise.
Since Tatsuya used Gram Dispersion, the psions that made up the magic sequences are still there; the difference is that they no longer have structure. With this large amount of psion noise remaining, from both the magic sequences on the Eidos and the psions generated by the CADs, Miyuki and Juumonji cast their magic to stop the car.
What I don't understand is why it makes it any easier to cast after Gram Dispersion. Isn't there still a lot of psion noise remaining?
Another thing that's weird to me is the pseudo-cast jamming explanation. Does this mean that two magicians standing next to each other cannot cast magic at the same time, since their psions will interfere with each other? Does this mean that in order to cast magic, magicians must be socially distanced? But this doesn't appear to be the case in many scenes.
Finally, I'm under the impression that once a magic sequence is written to an Eidos, other magic cannot affect it. This was used in the explanation of why the English scientists' flying magic experiment failed. In this case, if multiple magic sequences are written onto an Eidos, only the effects of the highest interference strength spell will happen. However, that's not what was stated to happen. In order for Tatsuya to use Gram Dispersion, the magic sequences have to have been written to the Eidos already; otherwise, you can't use Gram Dispersion on a magic sequence that doesn't exist. So, it seems that even if nothing happened, at least one magic sequence would have affected the incoming car.
In addition, if Tatsuya's use of Gram Dispersion did remove the psion wave interference and the existing magic from the Eidos, then couldn't this have been used in the development of flying magic? The limitation of Gram Dispersion is that magic sequence has too much data to comprehend, but if one is casting the magic then one can also easily cast Gram Dispersion on their own magic, since they (subconsciously) know the full contents of the spell. in this case, I'm not sure why the English scientists' flying magic experiment failed.
