I recently came across a Pro Kennex 5G SMI (Secondary Mass Index) - and I cannot find any information on the SMI (Used 10 search engines on internet) and no ref on PK web site. I own a 5G (which I think is great). Any one know about the PK5g SMI? (Comparative weight - head balance) Is it as arm friendly as the PK5g? What does "Secondary Mass Index" mean? I would appreciate your insights.