My experience with the Merc LU shifter is that the relaxed position of the shift mechanism is that the spring loaded "cam follower" on the front of the prop shaft is the farthest forward, relaxing the pressure on the spring behind it on the prop shaft.
The cam follower follows 3 detents in the shift cam plate controlled by the splined shift stub sticking out of the front of the LU, each being a little farther from the pivot point as you go from F to R. Tension increases as you twist the shaft to N and increases more as you move on over to R as R requires the clutch dog to be "pushed" back to the rear to engage R gear.
Having had it apart surely you noticed that the clutch dogs faces and the rear of the respective gears are different shapes.
F gear and the front of the dog are sawtoothed and that sawtooth design is an "overrun" clutch allowing the prop to free spin when the engine is decelerating faster than the boat is slowing down when the boat is moving forward and you chop the throttle. It lets the prop "free wheel" or overrun allowing for a smooth transition rather than a "put your teeth in the windshield" abrupt drag...brake. R gear is usually straight sided, lock in place slots and cogs, as the overrun clutch isn't always a requirement as in R gear you are never going all that fast.
So with the shifter cam in the center (N) position, obviously the prop free wheels. Moving to R one has to turn the prop as you move the shifter to find a spot where the cogs and slots line up and the cam then continues moving in the R direction and completes it's travel equal in distance from the N position to that of F gear.
Moving in the opposite direction with the shift shaft, going through N and stopping in F gear, the spring tension on the shift shaft is minimum and the clutch dog is plugged into the rear of the F gear, both having sawtooth surfaces. Looking from the rear of the engine, turning the prop CW causes the prop shaft to move in and out slightly as the sawtooth surfaces rise and fall over the peaks and valleys of the gears and the prop shaft turns without the drive shaft (engine crankshaft) turning....that's the overrun clutch working. Turning the prop CCW it doesn't want to move easily as you are having to turn the whole drive line as you had to do in R (in both directions).
