By using this tricky spell a Mage may attempt to interrupt the flow of energy another Mage is using to cast a spell before they can finish casting it. Flow Disruption must be cast faster than the other spell to have a chance to disrupt it, but 1 round spells can still be interrupted if Flow Disruption is used at the same speed.
To interrupt the spell the Mage must be able to see the caster as they are casting. They must then cast the Flow Disruption with the base difficulty increased by the difficulty of the spell they are trying to disrupt (do not count power levels). If their roll succeeds, and has a better margin of success than the casting roll of the other Mage, then the spell is disrupted.
For example a Red Order Mage is casting Dustflash (difficulty -3), and has a base Magic Save of 15. He rolls to cast against a 12, and rolls a 7 (margin of success 5). Another Mage using Flow Disruption tries to prevent him. She has a base Magic Save of 14. She must cast Flow Disruption at -5 (base of -2 increased by the difficulty of the Dustflash), and roll less than 4 (margin of success 6 or more), to interrupt the spell.