- "He can control people."
His gift works mentally, like most others.
Theodore's gift allows him to compel people by speaking, and the victims are unable to disobey; the seemingly cogent commandment is far too compelling.
He is even capable of persuading people into hurting/killing themselves, but he can never cause a victim to do something they are not capable of doing. (Ex. Flying).
Due to the fact that Theodore's persuasion works by speaking, if someone cannot hear him, he cannot persuade them. Also, someone who is very strong-willed can resist him, though this is very difficult to do. Bella Swan and Alessandra Cipriano are able to shield themselves from Theodore's gift, as well as others.
As a human, Theodore's gift of persuasion had already manifested due to his gift of power augmentation. He was able to compel people to do whatever he wanted them to do.
When he became a vampire, his gift only became stronger. He was able to use his persuasion at longer distances and for a greater amount of time.