There is an announcement and a description after the performance. According to it, the piece takes about ten people to set up the metronomes to the exact specifications of the composer and in the past it required some quick work to set them going. Then more than thirty years after it was composed, someone who was fascinated by the piece came up with a means of setting the metronomes going automatically. That's what this video shows.
I haven't had one of those things in close to fifty years, I dropped mine and it never worked right after that. I went electric with an old analogue Franz metronome, I loved it because you never had to stop and wind it up but you couldn't play this piece with them. Nowadays it's all digital without the annoyingly familiar mechanical click.
It's been a long time since I posted a piece of legitimately avant-garde music. This one is fun.
Update: Steve Reich - Pendulum Music 1968