The old string method was part of my training in the 50's. The coach had it neck high, riding next to your neck and streached across the gym. As you shadow boxed and moved forward, you had to duck under the string just enough to get on the other side with your head, or to duck directly under the string and come back to your original head positon. You also did standing spar with your opponet throwing a continual left or right at your face, and you had to cover and move your head side to side, or duck.