The dead-band in a flight control is a region of travel where stick movement does not result in control surface movement: a zone where stick input is dead.
With hydraulic power ON, moving the stick opens a servo valve, upon which hydraulic fluid starts to flow from the actuator, which moves the elevator. There is a follow-up mechanism in the mechanical linkage that closes the servo valve again when the corresponding elevator position is reached.
With all hydraulic power OFF, the elevator needs to be moved by human push/pull power. But when moving the stick, it travels within the actuator servo valve band first. Only with the servo valve running into its end stop is there a direct mechanical connection between stick and elevator - but only in one direction.
So from neutral:
- Start pulling, nothing happens with the elevator (a dead-band).
- The servo valve hits the end stop, and now the pilot may feel the aerodynamic forces on the elevator, which they pull against further to create a nose-down input.
- The pilot then reverses direction and pushes the column, which moves the servo valve out of its pull direction end stop.
- Nothing happens until the servo valve hits the push direction end stop (another dead-band).
- And now the applied push force results in elevator movement in the nose-up direction.
There are more effects at work, like the airspeed, the speed with which the stick is moved, internal friction in the control loop etc.
The friction forces in the cable loop are considerable, the aerodynamic hinge moments are fed back to the flight control, and there is this dead-band - all making manual reversion quite an experience.