What is the purpose of the optional locking solenoid mechanism?
This is designed to prevent undesirable backdriving forces moving the valve without mains power. On loss of power the locking solenoid will be powered down and the spring-loaded locking mechanism will hold the drivetrain in position. A pin engages a slot in the gear train to prevent motion. When power is restored, the locking solenoid will be energised against the spring, once again allowing the motor to drive the actuator to the desired position.