Set a field value with FAPI states

Question

Does anyone know if there is a way to set the value of a field with states?

E.g. If "this" is checked, set input value to "0"

Something like....

$form['booking_process'] = array(
        '#type' => 'textfield',
        '#states' => array(
            'value["0"]' => array(
                'select[name="field_relation[und]"]' => array('checked' => true),
            ),
        ),
    );

I always do this with my own JS, just wondered if there was a feature in states I was missing?