When Stella first manifested and borrowed another's ability, she seemed unaware of it at first, until she noticed that Ebon had lost his biokinesis, and her own ability was identified later. She didn't borrow the ability deliberately, but it could have been triggered by the dangerous situation she was in at the time, since they were both caught in a car crash. By now, she can control her ability fully, and only borrows an ability when she wants to do so. She must have physical contact with a person to borrow an ability and to return it, and the person will lose the ability until it is returned. She could potentially remove the ability permanently.