I guess it's one way of looking at bow efficiency, energy out per amount of draw weight. Problem is that you work harder drawing say a 60 pound hard cam bow vs a 60 pound round wheel bow, and just going by draw weight doesn't tell you how much energy you're getting out for work put into drawing. The real way to figure efficiency is to determine the bow's stored energy and the arrow's kinetic energy. Then divide the kinetic energy by stored energy.

At least their way gives you some kind of number to discuss over the campfire. Going by their method, my Power Factor with 600 gn arrows is 1.02667. With 470 gn carbons it's 1.00213. 60 pounds at 33 1/2" draw.