Most recipes I've found suggest flipping the half-done tortilla from the pan onto a plate, and then sliding it back onto the pan. However, that would leave egg residue on the plate. Why not slide it onto a plate, place the pan over the plate, and flip it back into the pan? I was wondering if there were any advantages of the flip and slide method that I might not have been aware of, because this does comprise a majority of the recipes out there.
The best tortilla de patatas should be juicy and thick, so it must be cooked in a deep pan. It is physically very difficult to slide it from a deep pan to a plate. Because the tortilla is just half cooked, its bottom will get stuck and its top jumbled and eventually slip to the plate. The most effective maneuver is first flip it to the plate and then slide it from plate to pan.
Other vegetable spanish omelettes (spinach, eggplant, artichoke, green garlic) are usually thinner so you can first slide and then flip... or can even be turned over on the air with just a stroke of the wrist.