When the eggs are mostly cooked, set a flat, rimless plate over the pan. Don't forget to give the pan a good shake to confirm that the tortilla is loose.
Give the pan a good shake to release the tortilla. If it isn't loose in the pan, help it along with a spatula. The eggs will be a little jiggly and wet in the center, but the tortilla should slide around as a whole unit.
Find a flat plate that's at least as wide as the pan and has no rim. (In Spain, there exists a special plate whose sole purpose is to flip tortillas—what devotion.) To do the flip, you'll invert the tortilla onto the plate and then slide it back into the pan to finish cooking.
Holding the plate firmly in place, invert the pan so the tortilla falls onto the plate.
Slide the tortilla back into the skillet, pushing any stray potatoes underneath. Tuck around the perimeter to round and neaten the edges.
You can eat the tortilla right away, or chill it and have it for lunch the next day (a wedge on a baguette is how it's usually done). To my taste, a tortilla hits its peak an hour or so after cooking.