Oh, that's cool! I had to look up what you were talking about - that's fascinating!

Yay!

So Linotype was a way of setting type for printing, primarily used with newspapers because it was fast, and they needed speed.

The basic way it worked: You had a keyboard. Pressing a key caused a physical letter to drop down on to what was to become your - wait for it - line of type (Linotype). These were lead slugs with the letters molded. So you'd type out your line of text, then hit a level that would push apart the spaces so that the line was fully justified (flush to the left and right margin), then it would fuse the line into a single object and send it away.

Someone else would be taking these lines of type and physically putting them in the frame for printing, and they'd build up the page this way. Other techniques for adding photos/graphics, and things like large drop capital letters and such.

After printing, they would melt down all the lead lines from the linotype machine and mold new letters and add them back into the machine to be used again. Incredibly clever and complicated, but so much better than setting type by hand.

But ETAOIN SHRDLU comes into it because there was no backspace - no way to get the letter you hit out of the line of type. So if you made a typo, you'd want to 1) quickly identify that line not to be used and 2) get it out of the way quickly. The Linotype machines didn't use a QWERTY keyboard; rather:

Notice the left side with lowercase and the right side with uppercase. :)

But also, if you made a typo, one of the quickest ways to identify that line not to be used was to run your hand down the capital's first two columns: "Today in the capital city, protestirsETAOIN SHRDLU ETAOIN SHRDLU ETAOIN SHRDLU" - hopefully your person setting the type would notice the all caps and not use the line.

Well, in fact, you can find numerous examples now that you know what to search for, but here's one: