[fsckboy]

I think the other replies to you are good, i'll just add,

unlike BMI, waist to hip focuses on abdominal fat and the additional claim is that abdominal fat is "the worst kind" for health, i.e. (i'm not an expert, and i'm not going to research, but from memory) men tend to get belly fat, women tend to get hip fat, but women who get belly fat are at increased health risk in the same way that men are, heart attacks, etc.