Limited Magic Resistance states:
You are less able to resist magic than other magi. You gain no bonus from one of your Form scores to Magic Resistance rolls, though if you are caught without your Parma Magica, you retain a Magic Resistance of 0. You may take this Flaw multiple times, for multiple Forms.
When does a magi actually roll for magic resistance? In what situations is this applicable?
The passage you cited makes no mention of rolling, only that the magus with Limited Magic Resistance has a Form which does not increase his Magic Resistance against relevant spells.
Per ArM5, p. 85:
Hermetic magi have a base Magic Resistance equal to their score in the Form most applicable to the spell cast.
Further down the same page:
The Parma Magica, Bonisagus’s great invention, is an Ability that grants magic resistance that adds to the resistance arising from a maga’s Form scores.
Therefore, a magus with Limited Magic Resistance will have Magic Resistance of only (Parma Magica * 5) against one Form's spells rather than the normal (Parma Magica * 5) + Form.
