how the value of open in dropdown of react-bootstrap change?

Question

1,i read in the react - the source code of the bootstrap.But I feel very confused, the open value is how change?The following code

dropdown.js
Dropdown.propTypes = propTypes;
Dropdown.defaultProps = defaultProps;
setBsClass('dropdown', Dropdown);
const UncontrolledDropdown = uncontrollable(Dropdown, { open: 'onToggle' });
UncontrolledDropdown.Toggle = DropdownToggle;
UncontrolledDropdown.Menu = DropdownMenu;
export default UncontrolledDropdown;