How a Jake brake works - with VGT - Animation

Animation showing normal valve operation + operation when Jake Brake is activated (Compression Brake).
When the compression brake is activated, the Fuel injector does not fire as it normally would near the end of compression cycle.
Under normal operation the Exhaust valves would remain closed during compression until the end of the power stroke.
When the Jake's are on, fuel injection is cancelled, and the Exhaust is triggered to open just BEFORE TDC to release the compressed air in the cylinder. This allows for maximum resistance to engine movement during the compression stroke, without the compressed air's energy being able to assist the 'downstroke'.
The goal with engine braking is to INCREASE resistance to movement of the crankshaft and rotating mass. By utilizing the engines high CR, the crank is slowed. Opening the exhaust valve early removes the stored compressed air energy before it can assist rotation. An additional strategy sometimes used is to close the exhaust + intake valves on the would-be Power stroke as well which creates a vacuum , further impeding the movement of the crankshaft.
This effect can also be utilized by restricting an EBP or VGT turbo (increasing backpressure).
Yet another strategy linked in is increasing boost pressure during engine braking, which increases the force required for compression stroke and optimizes this type of engine braking further. This is very evident in this video utilizing a VGT