Well, I'd explain it by saying that it's mostly special training. Now, if you ask me, most of the limit breaks are completely nonmagical--even Irvine's crazy crazy Shot. A lot of the special effects in the battle system are just embellishments to make things more flashy, which you can see if you can compare the FMVs of combat to the battle system itself. Ice Strike at the end of Disc 1 against Ice Strike in battle with Edea look different, and so I take the more 'realistic' of the two.

As to how they do it--the nonmagical limit breaks just rely on an adrenaline burst and the skills of those involved. Zell beats the heck out of things (he's so /good/ at that), Irvine engages in rapid invisible reload action. I'm seriously not even going to talk about Rinoa and Angelo other than saying that someone needs to call the SPCA on that girl. (Dogapaults are not nice.)

But as for the magical limit breaks, I figure that it's something of the same thing. Garden students are trained in more than one form of para-magic, by implication--it's just that GF para-magic is the most powerful form, and so it's the one that they rely on. For their limit breaks, Quistis and (presumably) Selphie trained to use that adrenaline burst to power magic. In Quistis's case, the organized blues, and in Selphie's a just sort of 'I'll channel magic and come up with whatever' kind of thing.

...And wow Selphie would be crazy as a Sorceress. I don't think she necessarily /is/ one, but she definitely has the capacity to become one, if the situation ever comes up. Moreso than Quistis, in any case, but that's another subject entirely.

...The idea is still funny to picture, though, if way too easy for some reason. We'd just have to worry about the 'Create Rocket Launcher' spell.