I have searched, but FCOM apparently does not tell us now.
The general principle is that each actuator cylinder has a feed of pressurised fluid to each side of its piston. These feeds are controlled by valves. The valves have four positions: open, closed, return to reservoir, and closed circuit. To move the piston, one valve is opened to the pressure feed, the other valve allows fluid on the other side of the piston to return to the reservoir, so the piston is pushed away from the pressure side, thus moving the surface it is connected to.
1. Damping prevents vibration, fluttering and 'bouncing'. On a car, dampers prevent the suspension springs from doing the same things as the car travels over bumps in the road. (car dampers are often called "shock absorbers").
On the aircraft, in damping mode both hydraulic valves go to the closed circuit position, so fluid can move between the valves and thus allow the piston to move, but against the resistance of the fluid flowing through the pipework circuit between the two valves. This resistance provides the damping force while allowing piston movement. Damping still allows full travel of the piston, and does not need system pressure from the Hyd pump.
2. If both valves are closed, fluid cannot flow so the piston will be held in one position. The valves probably fail to the closed position if electrical power is lost. We are not told how the system sets the piston to the centre position before both valves close, but there must be some sort of interlock.
3. Again, we are not told, but any commanded rate of control surface movement beyond what a single jack can provide will automatically employ force from the other jack. There is electronic feedback of the surface position to all the associated FBW computers. I can imagine that the secondary FBW computer monitors the primary and assists if necessary - in normal operation the primary jack provides its full force and the secondary jack might assist with, say, 10% of its force for very rapid movements. If the first computer or jack has failed totally, the secondary computer will need to energise its jack 100% to achieve the required movement and thus will take over completely, providing the back-up function.
As I say, this is a general principle of how hydraulic control surface jacks work.