These Jaws allow better access between the chuck body and the rear of the workpiece. They are also very practical when pre-turned workpieces or long workpieces have to be refastened. For example, when turning serviette rings or remounting rough turned work.
 
The sizes indicated on the drawing are when the jaws form a perfect circle.  VM90 and VM100 chucks provide approx. 38mm of expansion beyond these specified sizes.