If you aren't familiar with two stage pistons, they make is so that your shock has different dampening rates depending on whether the shock is compressing or extending.. When the shock is compressing the piston maintains a higher dampening to keep the wheels from bouncing off the ground just like a normal shock. When the shock is extending however, it provides a lower dampening - which means your shocks extend quicker to get your tires back on the ground, and back to providing tractions as fast as possible.
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