Welcome to the site !!
What he is referring to is the Dogbone (the part that says Pro Circuit on it , that looks like a leg bone a dog would chew on) , usually they are longer than stock , what that does is help keep the rear up , by changing the arc of the linkage , giving more traction , and usually a plusher initial part of the the stroke ( when the shock first starts moving/squatting) , they usually help hold the bike in corners better , I have used them ( not the Pro Circuit one , but aftermarket dogbones) , they do help with some areas of handling issues , but not all , depending on your needs , but also depending on what bike it is on , will determine what it is being used to to remedy or alter , some are shorter than stock and some are longer ..... to notice what it does , you would better understand it by having a stock one installed , riding a specific track and get used to the bike and what it inherently does , then installing the aftermarket one , and ride the same track/conditions to actually feel the difference ....
It is basically one of those items that you don't notice what it does , till you ride the same bike without one , some people like what they change , some don't ..... they are not cheap ..... are they worth it ? , it depends ...... on one of my bikes , it was a game changer for the track (this was a Pro Moto Billet Adjustable Link) , the one I put on my trail bike helped because it lowered the bike significantly which made riding off road much easier (this one was a Kouba Link , which is specifically made to lower the bike and nothing else) when you need to dab your foot and the bike is not 12 ft in the air , , now on my new one , I have not felt the need for one yet .... , again , you need to ride a bike (preferably the same bike) without one , to notice what it does and decide if it is a improvement or not for your specific needs , they don't fix every suspension issue , but they do help sometimes .... but they are not a necessity usually , and as I mentioned
, they are spendy ....... Hope that helped .....
