The secret to a fluffy everyday omelet is a combination of air and steam. When you whisk eggs vigorously, you are physically trapping air bubbles inside the liquid protein. When the eggs hit a hot pan, the air expands. Simultaneously, the small amount of water naturally found in the egg (or added in the form of milk or water) turns into steam. This steam inflates the bubbles, while the egg proteins coagulate around them, 'locking' the fluffy structure in place. If you cook the omelet too slowly or over-whisk it too far in advance, the bubbles will pop, resulting in a flat, dense egg.