I remember the "shelf" references! I believe it was the workouts after the Classics, yet before the BSS series. Instead of flat, or bubblicious, some people's butt became more box shaped. Mine included! The top of the butt went straight out from the back, almost level and parallel with the floor, instead of nicely curving downwards.
I remember when one of the Firm instructors (I can't remember her name), led a FitPrime workout. And she even said to put your hands on your "shelf". I wanted to run in horror. No more shelf, please!
I agree with the cause. When I was developing a "shelf", I was using both stacked boxes and going "heavy". Now I stick with the 8" box & bodyweight is fine for me!