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Two strokes don't have much engine braking - dood.

 

Here your anwser:

A compression release structure for a motorcycle includes the customary exit valve communicating with the combustion cylinder interior and may be lifted from its seat by manual operation of a pull cable in order to expel combustion product and gases under compression from the cylinder. When the cylinder pressure is diminished to a point where ambient air would otherwise tend to enter the cylinder with its contaminants the air is prevented from entering and fouling the cylinder and lubricants by a spring-biased one-way check valve. The one-way check valve is self-centering and of conical shape to swiftly establish a positive seal with its seat and block out the atmospheric air.

 

Don't make no sense like this bunny with a pancake on its head.

 

Two strokes engine brake. How they work.

 

If you look at the bike they are 4 strokes engines on most of those bikes.