I asked him if he knew because it makes a very big difference in regards to what you can reach with the boom. An "over center" boom will move any where you want it to. Flip it over, full latteral extension with both booms etc. A "non over center boom" will only articulate up down with the first boom, and top boom up down to limited degree to hit the spots not directly over the top of the truck where an "over center boom" will lay both booms over as long as they are for reach. I'm doing a piss poor job explaining this I think.. I need some pictures. LOL.....