MvTriggerCount & PlayThread

I have problem understanding the process of catching keyboard or mouse events and the player's animations driven by them. in the default.bind we can see incrementing $mvTriggerCount# and other variables in the functions for keyboard and mouse events.How are they binded with the playthread functions, i.e. how the engine understands when the left arrow of the keyboard is hit to play the player's "side" animation?
Though I am interested in what number thread i have to play any additional animations , for instance, the player smiling while he is moving. I assume that by default the player uses thread "0" for "run" animation. then having in mind this do i have to call playthread(1,"smile") or i have to call this function with different number?2 or 3? how is this thread system working in fact?