1968 Case 580 CK Question "S" position

I have been restoring a Case 580CK Loader Backhoe, of which is going pretty good considering that I mostly work on old Fords, Worthington, Jacobson and Fergusons. That said ... I am boggled by the purpose of the "S" position on the transmission shifter. I have a power-shuttle/hydraulic clutch all of which works well as does the 188 diesel engine. I just cannot find any information for the use of "S" position on the transmission, when engaged I hear something powering up. Is this a flywheel load for the engine, when the backhoe is in use?

Thank you,

Peter-S