Enhance/negate spell

You will interrupt or enhance a creature in the process of casting a spell. If the creature is casting a spell, its spell fails and has no effect, or it is enhanced making its effect doubled. It does not get a saving throw.

  For example you see a caster casting a healing spell, you can have it negated without a saving throw. Or have its effect doubled. If it heals 4 points of damage. That would increase to healing 8 points of damage.  It does not increase range or duration.