AoEs, by default, deal -30% damage to their secondary targets and +10% on their primary. Every point of this skill additively increases these values by by +10% and 5%, respectively. You also gain +1 target on AoE powers 33/66/99% of the time. Every point of Controlled Burst negates 1 point of enemy Front Row for the sake of reducing AoE.
An increase in accuracy means the targets are more likely to take full damage. The second rank improves the Spread targeting method (added at the end of October). Rank three makes AOE powers hit more targets more accurately.