apply a pneumatic signal in clutch line 52 and in throttle line 48 pneumatic control valve 46 is of the type that actuation of a control lever in one direction will induce a clutch signal during initial movement and thereafter will produce a throttle signal the pneumatic signal in clutch line 52 acts through lines 54 and 56 on the actuator ports of stack valve units 70a and 70d pressure applied at the actuator ports plugs the input lines to stack valve units 70a and 70d thus the rig air from line 64 is plugged by stack valve unit 70a thus disabling the first mud pump control valve 12 which comprises the primary means of regulating the mud pump 6 from the driller 39 s console the pneumatic signal in line 52 is however transmitted to the second input port of shuttle valve 20 since there is no pressure in line 14 any clutch signal in line 52 at shuttle valve 20 will be transmitted through line 22 the pneumatic signal in line 52 communicating with line 56 also disables the throttle input to stack valve unit 70d isolating shuttle valve 34 from the throttle line 48 stack valve unit 70c however remains open and the pneumatic signal in throttle line 48 will be transmitted through line 61 to shuttle.