Over the shoulder is often the developer explaining their decisions in the code, instead of the reviewer trying to reverse-engineer it, independently.

If the reviewer needs to reverse-engineer the code to understand it without the author explaining it, this is a strong signal that the code is not high quality and needs more work. In this case, “over the shoulder” is actually bypassing something that a pull request would catch. “It’s not clear what this code does” is a perfectly reasonable cause to ask for changes in a pull request.