3/4 off their shoulder?

"Drafting is running behind someone, 3/4 off their shoulder, and it can save up to seven per cent of your race energy output by minimizing your air friction...Find a racer who is only slightly faster than you, and fall in just behind his or her shoulder. Maintain a courtesy distance of about 1.5 metres...."

Does "3/4 off their shoulder" mean that keep a distance of 3/4 shoulder length?How can a person's shoulder as wide as about 1 metre? What does this 3/4 refer to?

No, it means being at an angle to them, where only three-quarters of your body is behind them. If you want to see drafting in nature, look at how geese fly in a V-formation -- it's efficient, because the birds ranged at an angle behind the leader all benefit from reduced wind-drag.

Thanks! It's very clear!