I'll try to explain my theory:
-you have a normal card, with nothing on it,
-you insert magic inside it (and this what is difficult to understand, let's just say that the magic you insert in the card depends on your wisdom/nature/will and some things, that might also give an element to the card, for example, Aya is extremely fast, and is a bird somehow, so the magic she put inside is based on the wind element and has the shape of a whirlwind, or a fast cutting wind)
-when you want to use it, you do some incantation, or just concentrate, in IaMP, when someone uses a card, they just seem to concentrate without saying any incantation, so they unleash the magic inside it.
That the way I see things, everyone in Gensokyo can create his/her own card due to "basic" cards (with nothing on it) they can buy somewhere (I suppose, they don't fall from heaven) and the person who bought the card can tranform it into a weapon assuming this person possess magic (this is why patchouli got more spellcards in EoSD compared to the others, due to her knowlegde about elements and possesses natural ability to use magic, otherwise, because she seems cold, she would just use ice type magic.
Cirno has a hot temper, so if she uses a magic, it would be fire, but here, we assume cirno has an element, she is an ice fairy, she can't change her condition, so she used only ice, also, notice that she generally uses ice picks, or ice stings, that might explain her personnality.
Also, to summon more strong magic, in IaMP again, the character must sacrifice other cards to launch the attack, this is why I say the cards contain magic, because the character has his own limits, and can only use a certain amount of magic at a time, so the character liberates tth magic of others card to have the amount of magic necessary to launch the spell.
Another thing: if you store magic inside a card, will the magic remain until you use it or will it stay forever inside the card?
For example, Marisa stores magic in a card for a Master Spark, then she let the card there and makes another tomorrow, and the same scenario for about one year let's say, so she ends up with 365 Master Sparks (assuming she got enough "basic" cards).
Because there are magic inside the cards, she is free to just concentrate and use the 365 Sparks one after another, but of course that's not logic, because:
-if the magic was ONLY inside the spellcard, the winner would be the one having more spellcards than the other character, so if Cirno has 999 spellcards and Utsuho has 666 spellcards for example, Cirno wins, assuming they have the same level.
Then the cards becomes useless, and the owner must "create" other cards.
-If now, the user uses his OWN magic to use the power whithin the spellcard, it is more logical: Imagine Cirno got 1 spellcard and Utsuho got 1 spellcard too, Utsuho can use her magic more often and she wins, because she is more powerful, assuming the power of a god is superior than the power of a fairy
, of course, in this situation, the fighters will end up tired, because they use thier own energy to attack.
Here, the cards don't become useless, the owner can use them during an illimited period, because the power in the card remains, it is the owner, while using his magic, who "converts" the energy in that card and summons a spell.
so:
Magic coming from the owner + energy in the card = spell.
TO SUMMARISE (aka TL;DR)
-a spellcard is a the beginning empty,
-the owner of the empty card create a "code" depending of his will/mind/wisdom/personnality...,
-the card is created,
-the owner must use his own magic to convert the "code" into the spell.
This is also why we only see the Touhou characters with a few speccards on them, if they were planning to go on a dangerous and long trip, they would take plenty of them, and that won't be convenient (even if a card in light, it is possible to travel with more than 1000 cards on you in a bag for example).
Here, all done.
Alka-Seltzer anyone? :V
