On page 2-23 of your owners manual tells you
the light will come on when:
The engine coolant temperature is higher than 169*F (76*C),
you are going 15 to 19 mph (24 to 31 km/h) and
you are 21 percent throttle or less.
The guy who invented this feature needs to be shot. The only time it come on for me is when I am stuck in traffic, usually turning left, when there is no way I should be going to fourth.
Like one of the other posts, I don't shift out of first unless I am over 3000 rpms (which is like 21 mph). Which in reality does the opposite of the desired "helps you get the best possible fuel economy"
From what I understand, it is a 'plunger' that pushes the shifter over and locks you out of the 2nd and 3rd. If the stupid thing engages when I don't want it, I end up shifting like I'm going to go into 4th but I put it back to second as soon as the 'plunger' dis-engages