Counter throw

This was written for HD Remix but I’m guessing it applies for ST as well. I have a question about the section of this article called Countering throws after jump in attacks. They talk about something called a counter throw there, what the f is that?

You throw them on the reversal frame, that’s all.

It’s basically saying if you stay blocking low, you can’t throw them if they attempt to throw you, if you block low and have directional input facing back after they tick you, you can throw them during the reversal frame.

What does reversal frame mean here?

Okay, thanks.