Your assurance of perfect alignment. The jaws actually float to permit the cartridge to find its exact center in the die.
When a case is in the shell holder, it’s supported by spring-loaded jaws that contact the base of the cannulure for more positive extraction. The shell holder is also self acting, so when a cartridge is placed in the die and the handle of the Co-Ax Press is activated, the jaws can open and close on the cartridge case. When the handle is raised, the jaws automatically open and release the case.
An alternative for other calibers is the Shell Holder Adapter Plate for Co-Ax Press, though it does not allow a cartridge to “float” as with our “S” and “LS” Shell Holder Jaws.