I'm guessing here but the dual stage clutch engages/disengages the PTO at about half way of the pedal travel. Pressing the clutch all the way in disengages the transmission from the engine as well as the PTO.
Engage the PTO and see if you can regulate it with the clutch. Take the tractor out of gear during the test of course.
It was very useful in using PTO equipment on my L3650.