Its definately a kickdown switch... i had the same in my 120Y before i converted it to manual.
It doesnt drop down a gear when ur going fast...its more when the engines underload. when ur climbing a hill for example.... U notice the ur speed drop down and Revs dropping also... The swich activates the kickdown solenoid, dropping the box down a gear, eg. Ur going up a hill in 3rd gear, at 60kms, the hill gets steeper, so u put ur foot to the floor, to accelerate harder., because the gear ratio is not right for the hill climb, The switch activates the solenoid, the gearbox drops down the 2nd gear to cope with the drop of RPM as the speed of the vehicle slows....

I hope this gives u a better understanding of the kickdown switch...

Also my 120y (Aus spec) had the kickdown solenoid on the pedal aswell, that is before i converted it to manual.