Fast Cast Defense

Can anyone explain this to me a lil better it seems pretty glossed over for sometin that could be pretty important.
If I can match half the level of a spell I can negate its effect on my wizard is that right?

Sort of, if you cast a spell with an effect that can reasonably protect you from the spell cast at you then you are protected. Most often this is pretty clear. In the situations where it isn't clear, use 1/2 the level of the spell you are defending against as a guideline.

if you know the form you can do it with 1/2 the level to deflect it. The grog standing next to you would get hit if it were and AOE attack. To negate the spell you need the level or greater.