I never worked in the theatre, but I know in ballet we were taught how to curtsy. Our instructor counted so we did it together.

Maybe they are counting to bow at the same time? Often they hold hands when they bow, and then they would all pull down together I would think.

I’ll be interested to see the answers.

When the cast comes out for a final curtain call, they usually rely on one of the leads, or stars to indicate when the group bows. It is usually through eye contact.

@Yetanotheruser That’s the same way I have always done it.

Watch how a string quartet hits the first note when they play. The first violin makes eye contact with the other three and gives a small nod. Similar to how they end the piece, which often has some ritardando and dramatic change in dynamics.

I have played two-handed piano with a friend and I give the nod to start. It is less clumsy than counting out loud (which also works).

An interesting 2012 movie about a string quartet (starring Philip Seymour Hoffman and worth seeing) called A Late Quartet
shows a number of vignettes about how they launch themselves.

@gailcalled I have played two-handed piano
I think you mean “four hands”. Most pianists use two hands!~ :P!

^^ Correct. My two hands and my friend’s two hands on one keyboard.

A quickly shared hand squeeze.

That’s why they’re always holding hands during the applause.

Interesting theories, thank you. On retrospect, with the cast I saw, I’m pretty sure they did the hand squeeze. They were all looking straight head, but there was definitely some movement going on with their arms.