Chickenplucker,
Because the knife is pretty fat to begin with at .250 The hollow grind is there for 3 reasons....
1) Increase slicing performance yet retain strong tip strength because of the full convex belly and tip
2) Weight reduction
3) And most importantly...it looks cool <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />

Nothern,
Thanks for the compliments. I think you like the BT4 because it has heavy Busse influence. Although, I never learned from Jerry directly...I consider him somewhat of a teacher. Modding alot of Busses definately influences some of my designs.