The 737 (all versions thereof) is one of the few jetliners still in production to have any manual-reversion capability for its flight controls in the event of the loss of all hydraulic systems. If hydraulic pressure is lost in both the A and B hydraulic systems, control tabs on the elevators and ailerons unlock, and the elevator and aileron control cables, which are directly attached to these tabs, deflect them to produce aerodynamic forces that move the elevators and ailerons in the desired direction.1 When hydraulic pressure is available in either of the two primary systems, the control tabs are locked to their respective control surfaces and move with them.
Thus, the control tabs - which are still attached to the control cables even when locked to the elevators and ailerons under hydraulic power - deflect in the opposite direction when the flight controls are being moved hydraulically compared to when in manual-reversion flight. For instance, when a pilot commands up elevator in hydraulic flight, the elevator actuators move the trailing edge of the elevators, and of their control tabs, upwards, even as the control cables themselves are trying to pull the trailing edges of the elevator control tabs downwards (as would be necessary when commanding up elevator in manual-reversion flight). How does the 737’s flight-control system prevent the forces generated by the hydraulic flight-control actuators from feeding back through the control cables (via their attachments to the elevator and aileron control tabs) and forcing the pilots’ yokes in the direction opposite their control inputs, or, alternatively, the forces exerted directly by the control cables from impeding the control surfaces’ motion when under hydraulic power?
1: Not all of the 737’s control surfaces have provisions for manual control; the spoilerons (which deflect asymmetrically to assist the ailerons in rolling the aircraft) are inoperative if the A and B hydraulic systems both fail, while the rudder (which provides yaw control independent of roll) can be powered by the third, standby hydraulic system2 if both the A and B systems fail, but does not have manual-reversion capability in the event of a failure of all three hydraulic systems.
2: The standby hydraulic system (which can provide hydraulic power to move the rudder, actuate the thrust reversers, and - if the ALTERNATE FLAPS switch is engaged - extend the leading-edge high-lift devices) is normally unpressurised, but can be manually activated by the pilots (and activates automatically in the event of a hydraulic failure during critical phases of takeoff and landing); this is generally done in the event of a failure of one or both main hydraulic systems, but the standby system can be engaged even if both the A and B systems are fully operational (useful for clearing some types of rudder jams, although less so nowadays than was the case with the 737’s pre-2008 rudder-control system).