Edit - This answer is no longer accurate for the current version of ACF, see [their documentation](http://www.advancedcustomfields.com/resources/#actions)

Use the `acf_save_post` hook.

	function my_acf_save_post( $post_id )
	{
		// vars
		$fields = false;
 
		// load from post
		if( isset($_POST['fields']) )
		{
			$fields = $_POST['fields'];
		}
 
		// ...
	}
 
	// run before ACF saves the $_POST['fields'] data
	add_action('acf_save_post', 'my_acf_save_post', 1);
 
	// run after ACF saves the $_POST['fields'] data
	add_action('acf_save_post', 'my_acf_save_post', 20);