exhaust horse power

well its 4:30 am and i cant figure out why when you let off the throttle the jake brakes work. i know jakes open the exhaust valve on the power stroke  thus making the engin use horse power to slow the truck instead of speeding up. The jake itself is made up of springs and valves that are controlled by oil pressur. To get to them the valve cover comes off on a isx cummins they will be on the exhaust side on the rockers there will be three of them," power exhaust and injector". the  exhaust you will see a round thimbel with a snap ring around it. when you press down on that it willrock back slightly when you press foword it will rock foword like an inch that is the exhaust  valve. if you ever take the snap ring off the internals are simple litterly two springs and a shaft.