How does Expose Weakness work?

From my understanding the Expose Weakness mastery means that whenever you damage an enemy champion, allied champions deal increased damage (by 3%) to them for the next three seconds. Is this correct? If so, how does this interact with damage over time abilities such as Teemo's Toxic Shot? Since the damage-over-time on Teemo's E lasts for 4 seconds, does that mean that autoattacking an enemy champion causes them to take an increased 3% damage from allies for 7 seconds (4+3)? Thanks in advance for any information you can give. ^_^