It's so funny! The orchestra doesn't play. They sit there quietly.
The conductor gives the downbeat and he uses a clock to time it.
You can hear all the coughs and you can see all the people looking at the orchestra.
Here's a recording of it:
I did find out that tacet means "It is silent".
I wonder how many bars each movement is?
What key signature is it written in?
Does the orchestra have to rehearse it?
Here's what John Cage says about 'silence'