Here is the set up:
When a wizard casts an evocation attack he decides how much damage he wants the spell 'weapon to be, right? So say he decides to put 7 shifts of power into it - making it an effective weapon:7 attack. He can control that much due to ability and bonuses to specialization and focus items, so he only takes the one stress. He then needs to make a discipline check to control and target with. Let's say he has a +4 on his discipline and rolls +3 on the Fate dice, for a total of +7. The target rolls not so good and only come up with a +3 total to defend against it.
I agree with devonapple's answer. Just to throw in a different twist, however, lets say that the spellflinger rolled only +0 on the dice, which means he had a net of +4 against the target's +3. In this case,
he still hits the target with one shift to spare, but he has failed to control the spell to the tune of three shifts (he needed to control 7 shifts of power, but only managed 4).
So now he has a choice. He can either accept it as backlash, in which case he (the wizard) takes a 3 stress hit (physical or mental, player's choice), and the spell is considered fully controlled and resolved. In this case, the target would get hit by the 'full' 8 shifts of damage (7 from the weapon rating of the spell, and 1 from the net shifts on the attack roll). The other option is to go with fallout. In this case, the weapon rating of the spell is reduced to the amount actually controlled (weapon 4) and the target gets hit with the resulting 5 shifts (the reduced weapon rating plus the net shifts on the attack roll). The wizard doesn't take any additional stress (maybe), but the GM gets to choose what form the 3 shifts of fallout take. If this was Dresden and the spell was a Fuego! spell, then the building would be on fire.
Not the question you asked, but it is a likely result for a discipline 4 wizard casting a power 7 spell, so I thought it worth mentioning...