what does this do???

 

I think the word "damper", is a more descriptive term. What a good steering damper does is to dampen the shock transmitted thru the steering stem into the bars and finally into the rider's arms and body, which on long or vigorous riding increases the fatigue of the rider. Most dampers are adjustable according to the rider's preference and also to the specific terrain he is riding. Some are adjustable at the bars, allowing the rider to adjust damping on the fly. Others are adjustable at the damper itself, such as the Denton, PEP, etc.
Some quads have a problem with head shake at high speeds and I had that problem on a Warrior I once had. The damper I installed eliminated the problem.