Koku's voice is lined with an electromagnetic pulse. If someone who considers Koku a worthy leader hears his voice, then their physical and mental state will be significantly enhanced. The louder Koku's voice is, the more effective his Quirk becomes. Using this power to strengthen someone else and their Quirk takes a toll on the recipient's body.
Monikered "Trumpet" for his loud influence, Koku uses his Meta Ability to increase the prowess of his followers in the Liberation Army. He speaks from his political campaign van and rallies his followers against specific targets. Mr. Compress compares him to a preacher leading his flock.