a backcut is a different angle (in your case 30*) cut from the normal seat angle ( IYC prob 45*) It goes from very near the edge of the seat inwards till it goes into nothing into the head of the valve( hope this is clear) It is done on a valve refacer. It will allow more airflow as will paying very careful attention to the valveseat area specifically as Dave has stated.I would also add since valves are "free" in these specifications you may consider thin stem valves also.