Operating Diesel Engines

Engine brakes have not always been around, in fact they came about almost 65 years after the first successful operating diesel engine. And since their introduction, they have become a staple in larger diesel engine applications.
The engine brake came about from a rocky start dating back to 1931, when Clessie L. Cummins and his crew had a near death experience. This experience led him to start thinking about how to use the engine as both a means to propel a vehicle uphill and to be able to control it better on steep down grades. In 1957, the idea to use preexisting components, but modified to open the exhaust valve came to him. The idea was so unprecedented that it actually allowed for a broad yet powerful patent to cover the idea. (About Us: Jacobs Vehicle Systems)
Early versions of the engine brake, were crude in their use of the injector motion to accomplish the goal of retarding the engine. And eventually it was realized that hydraulic operations of the system would be much more efficient and practical. These internal engine retarders, more commonly known as compression release brakes, operated on the idea that to control power and slow the vehicle, the engine could vent off the compressed air before it was ignited. And help slow the engine by cutting out a power stroke. The downside to this is that the exhaust becomes very noisy and is thus banned from being used in most cities and towns.
The actual engagement, or activation of the engine brake is very simple. Once the diver has turned the system on by a switch, solenoids activate and lock out the cylinder valve stem. Thereby allowing the valve to be opened by the cam and vent the compressed air out the exhaust. Several systems incorporate a switch that not on...

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...the crank rotates, it transfers power to the housing vanes. Which are located on the housing of the unit, thereby creating a fluid drag, which absorbs power and slows down the vehicle. (Truck Troubles.com) Activation of this style of retarder is accomplished by filling the chamber which holds the rotor and housing vanes with oil. With the chamber filled, the rotor and vane become hydraulically engaged and slows the engine down. As the engine slows down a valve regulates the amount of oil into the chamber to help maintain a constant braking power, which is based upon either the drivers input or goes to maximum when the clutch and accelerator are released. The oil used in this system is pressurized to control how much braking power is applied, higher pressures result in higher braking. Whereas lower pressures result in lower breaking power. (Truck Troubles.com)