How the Tempo on timeline works?

it looks like an easy question which i should found answer everywhere, but no, it drives me little crazy. so the idea is super simple
playback a movieIn TOP speed based on the tempo.
when i change the timeline tempo from 120 to 20 , the speed never bring down. :blush: :blush:
how did this timeline tempo works?
do i need to convert the tempo into the playback speed of the movieIN TOP?