When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The round thingie goes down into the jack pad, keeps the board centered and prevents it from sliding off the jack when positioning under the car. The little clips on each side center it on the crossmember and keep it centered front to back. Making your life less complicated it's "dndrsn-ized".
They really are a work of art. When you're not using them for jacking the car thay make great paperweights
Mind you, I now have the rocker rails fitted and they cover the jacking holes. I tend to jack using the jacking point on the frame. I may just make up one of those boards