trigger-block

from The Century Dictionary.

• noun That element of the mechanism in a Corliss valve-gear by which the valve-stem is released from the rod which actuates it, permitting the valve to close suddenly by gravity or a spring or both. The trigger is released by a push or pull from a rod connected to the governor of the engine, whereby the speed is made to determine the point at which the trigger is pulled, and the admission of steam cut off.