Penjelasan detail mengenai 'Flashdata' di CI beserta contohnya

contoh nya:
controller "function login"
function checkLogin(){
$username = $this->input->post('username');
$password = $this->input->post('password');
$valid = $this->MUser->checkLogin($username,$password);
if($valid){
$this->session->set_userdata('user',$username);
redirect('site/home');
}else{
/* jika validnya false dia akan lempar ke form login lagi nah disini lo set error untuk ditampilkan diform login incorrect */
$this->session->set_flashdata('error','incorrect');
redirect(base_url());
}

}

view : form login

blblbl
blblb
blblb
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//untuk manggil incorrect di controller tadi
 echo $this->session->flashdata('error');?>