BBT40 Holder Taper and Both Face are in Contact with Spindle due to Which Spindle and Holder act as Single Unit this allows a Highter " Radial Rigidity" hence we get good surface finish and dimensional accuracy while maching also low vibration at highter speed which increase tool life. 

So These are better than general BBT40 Holders but it should only be used with BBT40 Calibarated Spindle Only.  

Side Lock Holder Is Balanced to G6.3-15000 RPM