If you define a beforeFilter in your controller, it will be executed before each action. Sometimes, that behaviour is not desired, and you want to exclude an action from the application of the beforeFilter. The following simple example gives you an idea of how to accomplish that:

// app/controllers/users_controller.php
class UsersController extends AppController
{
    function beforeFilter()
    {
        if ($this->action != 'login')
        {
            // execute beforeFilter logic
        }
    }

    function login()
    {
        // do login
    }

    function edit($id)
    {
        // do edit
    }

    function delete($id)
    {
        // do delete
    }
}