Tortilla de patatas: Flip and slide, or slide and flip?
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.