Lever down = MP Low. Lever up = MP High.
MP is an auxiliary hydraulically operated clutch that changes the normal 6F/2R tranny to a 12F/4R.
You can start the tractor in either MP Low or MP High.
The MP can be shifted on the go from one range to another without using the clutch. Always move the MP lever all the way up or down when shifting.
MP can be operated continuously in either range.
MP High provides engine braking when descending a hill. This is one test to verify that the MP is working in High range.
MP High has a hill-holding feature when moving uphill. Depress the 2-stage clutch through the first stage only (clutch pedal halfway down). The hill hold will keep the tractor from rolling backwards (without the need to press the brake pedal). Hill hold works in both forward and reverse gears but only with the engine running. This is the other way to test the MP.
If there's difficulty shifting the standard gears on hills, put the MP lever in Low range.
You cannot start a tractor with MP by towing or pushing.
Good luck in your quest.