Did they give any explanation as to why/how being softer (does that only and strictly mean more relaxed?) in the upper body (which parts specifically and under what functions of those parts?) caused a more profound impact on the attacker, and what that impact is?
Being relaxed in the upper body is a direct result of the lower body and middle portions being able to take the load of the upper body onto itself. In otherwords, if they're not conditioned enough, you won't be able to relax the upper body.