2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 290Get this manual
4) SHIFT LIMITER Shifting to Forward or Reverse from Neutral is only allowed when the throttle is in the idle positionThis is safety device to protect sharper startingWhen the throttle is opened, the throttle arm (1) is rotated, and the cam roller pushes the plunger (2) outThe plunger blocks the rotation of shift link arm (3), causing no shifting(1) Throttle arm
(2) Plunger While Idling
(3) Shift link arm