It depends on what you plan on doing.
-If you use it as defence, you (or any person you protect) add 4 dice (or 6 in combat) to whatever you have to use to counter the spell (willpower, body, ....)
-if you use it to dispell a sustained spell, you have 9 or 11, as that's magic + counterspelling vs spell's force+ caster's magic.
you can find more info in the core book (20th aniversary edition p185).