You are correct up until here:
Which means, if I make that roll, it's automatically considered a twelve-shift attack on the other guy? Is this correct? Does the amount of power ALWAYS equal the difficulty of the roll?
If you make the discipline roll, it will be a 6 shift attack with a weapon:6, not a 12 shift attack. If the defender reaches more than 6 shifts on his dodge roll, the weapon:6 spell does not do anything to him.
The discipline roll serves 2 purposes: targeting and control of the spell. Let's look at them independently.
Targeting:
Much like with a regular weapon, your roll is the target number for the target to dodge. The spells power does not factor in here in any way.
Control:
Unlike a regular weapon, you create your weapon with the same roll you use to target it. You decide what power the "weapon" will have
at the most at the time you attack with it. If your discipline roll matches or exceeds what you set for the spells power, you can proceed like any regular weapons attack. If your discipline roll is below what you set the power of your spell, you will have to make up for the difference in 1 of 3 ways:
- invoke Aspects: This is the best way to go, if you have the option. It raises your roll to match the spells power and you can proceed as above.
-Backlash: You take an amount of stress equal to the number of shifts your roll was below the spells power. This will make the spell work as intended.
-Fallout: The excess power of the spell, that is the difference between your roll and the power you chose, will be let out randomly, and usually to your disadvantage. The details are usually set by the GM.
After all of this, the defender rolls an appropriate skill to dodge. Usually that will be Athletics, but stunts might change that. If the Athletics roll succeeds, the spell misses and the defender does not get any stress. If the dodge roll fails, the number of shifts are still subtracted from the attack roll, and the remaining shifts + the weapon rating of the spell are the resulting stress.
If the defender has a block or armor active, it will still reduce the stress of the attack by its value.
Example:
Let's take your Discipline 3, Conviction 4 mage who does a weapon:6 fireball.
First he takes 3 shifts of mental stress, like you said above.
He rolls a 5 on his discipline roll, leaving 1 shift uncontrolled. Since he does not want to take any more stress for the spell and doesn't have an aspect he can use, he decides to let it out as fallout, setting the room on fire.
His target now has to dodge a 5 shift attack, but with athletics at 2 and a roll of +0 he fails to do so, resulting in 3+6=9 stress. Luckily, he is a wizard himself and put up a block:5 shieldspell in advance, so he only receives 9-5=4 shifts of stress.
If you have an aspect to invoke, it can only improve your discipline roll, not your conviction to control more power safely (simply because it isn't a roll). On the other hand, with enough aspects to tag, you will get something along the lines of a 12 shift attack with a weapon:6 spell, which is bound to hurt regardless.