(SOURCE)

It’s pretty much just “ready, set,” and then instead of ‘go’ it’s an “ey!” type of sound effect

I saw people translate it as "I said no", too.

that has to be machine translation, shitty one too.

It's on the source video, so, probably a dumb person.

の and で are particles. It doesn't make too much sense have both right next to each other, but think of it as "(一斉の)で" i.e. "on 'issei no'".

えい！is the Japanese way to shout when trying to put their back into something. You'll hear it in Karate often.

Literally it means, "When I say 'together;' here we go!"

It's kinda funny, because it's like saying "on three," but not actually counting to three because "on three," itself is the count.

"Okay, here we go!" would be the next one then?

No, even though what I said I'd the literal translation, it works as "okay here we go."

Thanks :)