CakePHP displayField usage


can you help me how to use CakePHP's displayField directive, I can not figure out how to use it...

so, in a model file, i have following code...


class Task extends AppModel {

var $name = 'Task';
var $displayField = 'projectName';

//The Associations below have been created with all possible keys, those that are not needed can be removed
var $belongsTo = array(
    'User' => array(
        'className' => 'User',
        'foreignKey' => 'user_id',
        'conditions' => '',
        'fields' => '',
        'order' => ''


how can i use this, to display fied projectName in select form field?

thank you in advance!!!