Accumulators are used to improve the ride quality on a car with hydraulics installed. They were originally designed to dampen shocks on industrial hydraulic systems but work very well in the automotive lowrider world.
To be most effective accumulators should be placed physically as close as possible to the place or component where the shock is generated (the cylinder), however in the short distances being considered in hydraulic installations on automobiles the difference is minimal and they can be installed inline anywhere between the dumps and the cylinder. It's usually easiest to put accumulators in the trunk directly off the dumps. One accumulator should be used for each cylinder. View the oil flow with accumulators diagram for more information on installation.
In order for the accumulators to soften the ride of the car there must be fluid running through the hydraulic system. A fully dumped car won't have any oil in the lines and thus there's nothing being sent through for the accumulator to absorb. Also, if the system is completely locked up (i.e. there's so much fluid and pressure in the lines that the accumulator is full and can't push back) they will be ineffective.
Accumulators will slow down the reaction time of the hydraulic lift and drop action. On very high-voltage systems the difference is minimal.
The gas charge is usually nitrogen and you can see when the oil is evacuated the inner bladder that holds the charge pushes the accumulator to a normally closed position.