What is the secret level in 868-HACK?

In the latest post on the game by the author, he mentions a “secret level”. What is the secret level and how do you access it?

Now the single-run scores are dominated by scores achieved with the secret level, much higher than what could be achieved with either version of .SCORE.

https://mightyvision.blogspot.pt/2017/04/868-hack-update.html

There are a few YouTube videos of folks “visiting” the secret level, but not only do they not explain how, I can’t even tell which part of this is the secret level!