The trans works fine without a downshift/kickdown switch. The purpose of it is to let you manually downshift when you "floor it" while driving for a quick acceleration. In the past, it was often referred to, somewhat incorrectly, as "kicking it into passing gear".

I'm not sure how it physically looks on a '69 but on the '68 it is a switch mounted near the carb that gets activated physically when the throttle is wide open.