An apparatus for pneumatically amplifying the force acting upon a lock rod to retain a toolholder within a toolholder receiving member has a spindle extending from the toolholder receiving member and at least two in-line dual acting piston/cylinder modules within a cavity within the bore of the spindle. Each module has a piston within an associated cylinder and wherein the lock rod, through a drawbar, is attached to the piston of each module such that when each module is pressurized, the combined force of all of the pistons is applied to the lock rod. The invention is also directed to a method for achieving this task.