That is very interesting. It sounds like the engine mounted pump runs the power steering and possibly the FEL but the disconnect is for the pump that runs the backhoe?
I'm really not familiar with this tractor.
The Jinma 284 has a mounting point for a rear mounted pump just above the PTO shaft. It turns with the engine speed and the PTO is run independently. However, the pump shaft is powered through the PTO (second stage clutch) input so if you push the clutch in far enough to disengage the PTO the rear pump will stop too.
My PTO has dual speeds and a neutral position on the speed selector. The engage/disengage is on a separate lever.
Does the pump that runs your backhoe come off when the backhoe is removed?