We all have heard dialogue like this:
"Scene 12, Take 3.....
sound, roll camera.....(clack)...(pause)
Action!"
So, what is the clack about when the assistant smacks the hinged top down on the slate. I asked the Director of Photography, Rachel Cushing, exactly this question.
The answer is instructive about the technologies of movie making:
The sound recording system and the camera are independent. So, the sound of the hinged top smacking down right in front of the rolling camera makes it possible for the editor to later synchronize the visible signal with the sound signal to merge these elements together.