In literal Russian translation reading something in a diagonal direction (читать по диагонали) means reading quickly, without digging into it. I think proper English translation would be "to skim through".

It means in a cursory manner.

edit: funny to see all that we all replied at the same time. Just to add to the conversation, "diagonal reading" is not just a Russian idiom, it also exists in French :)

In German it's "querlesen" (read s.th. transversely, read s.th. across).

I guess Russian and French people start top left and stop bottom right, while Germans start top center and end bottom center ... (at least that's what I do).

In German we say „einen Text Überfliegen“ - fly over a Text. Funnily a „Überflieger“ is a person who excels at something, maybe with low effort.

Seems like a literal translation of Russian idiom meaning “skimmed through the text”.

I took it to mean "unofficially and as an outsider".