Rolling a Spell Check
Spells may fail if the target actively resists. The victim may make a opposed check vs. the spellcaster to resist the spell, unless it says otherwise. The spellcaster and the victim both use the attribute as prescribed by spell's type of magic for the roll. This is called a spell check. Success means the spell fizzles out and does not affect the target. In this case, the MP spent to cast the spell are lost.
Some spells specifically require you to concentrate for the entire duration of the spell, or else the spell ends and your victims are released from its effects. Other spells may imply that concentration is necessary, but from a mechanical point of view these are directed spells. For example, a spell like Ensorcell specifies concentration explicitly, whereas a spell like Control (Gravity) only implies it.
(See the rules on concentration in spell logic for more details.)
Resisting Ongoing Effects
When you're subject to a spell that renders you helpless or otherwise incapacitated, for each round you're subject to the effects of the spell you may make a spell check to break the spell's hold over you.
Each round, the spell check is made at an escalating +2 against the spellcaster's original TN. Therefore if you're subject to petrification, on the first round after you've been petrified, you may attempt to resist at a +2. On the second round, at a +4, and the third round, at a +6, and so on.
If the spellcaster is actively concentrating on a spell against you, they are entitled to re-roll the spell check to set a new TN on their turn.