If your document is in portrait mode, you have to use flip over i.e. long edge and if your document is in landscape mode, you have to use flip up i.e. short edge.
As the printer needs to flip the sheets to duplex, the edge to flip them over is the short edge. If they were flipped along the long edge, the back of the page would be upside down compared to the front.
If it is printed on a double-sided printer (common in offices), it'll come out correctly if set manually by the user to "Flip on short edge", but will come out with the second page upside down if default settings are used (flip on long edge).
When I turn page, I turn horizontally like this picture. This means I pick "Flip on short edge" right? Pls see screen shot below.
For the 'flip on x edge' question, it depends on the layout of your booklet. If your booklet is A4 sheets folded (and so each page of the booklet is an A5 page, 4 per sheet) then you want 'flip on short edge'.
printing - Create a PDF that defaults to flip on short edge when ...
Therefore, for a correct double side printing, I would need to 'flip on the short side'. However, I have a machine that only flips on the long side (because it's an on line printer, for example). So, in order to print correctly the book, I would just need to rotate 180 degrees clockwise every second sheet. How can I do this?
Printing 'flip on short side' on a 'flip on long side' printer