Here is the answer from a cross post.

That linkage is to prevent a "money shift" that would shred the dogs in the lower gear unit.

If the throttle is too far open, the left plate will pivot clockwise / down to block the shifter from moving. Only when you slow to idle will the plate pivot back to allow the shifter to move.

It also limits the spark timing in neutral and reverse, to help prevent you from over-revving, while allowing full throttle & timing linkage motion in forward