This makes sense
Found this from, of all places, the Traxxas website. Pretty straight forward:
"This adjustment is made by changing the position of the upper suspension arm pin location. There are two positions available. The upper position allows more chassis roll and basically eases the weight transition of the chassis. The lower positions "tighten" up the chassis and resist chassis roll, kind of like a sway bar. The result is faster weight transfer, which makes the chassis more responsive to input. Since the Slayer's track width is fairly narrow compared to its wheelbase, the chassis is inherently responsive. I recommend starting with the roll center pins in the upper holes. Only use the rear pin locations as a possible adjustment. I recommend not using the lower front roll center location, since it makes the front end a little twitchy. It will also take out a considerable amount of steering from the chassis. The rear is different. The lower pin position can actually be used when extra steering is needed. However, if you're already using the Slayer sway bar kit, I recommend leaving the rear pins in the upper location."