WardL,
Visualize a cylindrical tank with a piston inside it. On one side of the piston is a charge of compressed air. On the other side is an opening that is plumbed to one of your
oil lines. When the engine cranks and
oil pressure begins to build up in the lines,
oil is pumped into one side of the tank and begins to push the piston against the air. When the oil pressure stabilizes at say 50 PSI, there is an identical pressure in the air charge trying to push the oil back out of the tank and into the oil lines. If the engine were to lose oil pressure briefly (e.g., during a prolonged hard turn when oil sloshes away from the oil pickup), the oil in the accumulator would be pushed back into the system to slow the drop in oil pressure. Then, when the oil pump resumed pumping oil, the accumulator would be recharged for the next time.
If you put a valve on the opening to the accumulator, you can store up oil under pressure by closing the valve before you shut off the engine. Then, if you want to pre-oil the engine before starting it, you can open the valve to release the pressurized oil back into the system a moment before starting.