Yeah, I'm going to have to agree with Kuja on this one. Magic takes time to cast, summoning takes a ridiculous amount of concentration and willpower to maintain. However, since characters are not vulnerable to attack while the summoned creature is present in any FF game, I'm going to say that the character is either protected by an impenetrable energy field or they just disappear entirely. I don't really care which is is, just so long as the summoner is invulnerable.