Why searing a steak is a heat transfer problem
Contact, moisture, and why the pan temperature drops.
Searing a steak is a heat transfer problem because the moment cold, moist meat touches a hot pan, it draws heat away from the metal directly through contact and through evaporating surface moisture faster than the pan can necessarily replace it, and a pan that cools too much at exactly that point never gets hot enough, for long enough, to actually sear anything.
Two drains on a hot pan
Placing something cold and damp on a hot pan draws heat out of the metal through two channels at once. Conduction pulls heat straight into the meat wherever the surfaces touch, and moisture on the meat's surface absorbs a much larger share as it flashes into steam. Five grams of surface water, about a teaspoon, needs roughly 11 kilojoules to turn to steam, enough on its own to take around 24 degrees out of a whole kilogram of cast iron.
Whether that drop matters depends on how much heat the pan has stored and how fast the burner can replace it. A heavy cast iron pan carries a large reserve in its own mass, so the same loss barely dents its surface, while a thin pan on a weak burner can fall so far that the meat's surface steams or stews instead of searing. A thin film of oil helps the conductive half, filling the microscopic gaps between meat and metal so the contact is more even.
A damp cloth on a radiator
Pressing a damp cloth against a warm radiator produces a faint hiss as the moisture flashes into vapour, and the patch under the cloth cools enough to feel with a hand held nearby, until the cloth dries and the radiator climbs back to its running temperature. The radiator never stopped producing heat. One spot simply had a demand placed on it faster than the radiator could resupply it. A dry cloth pressed against the same radiator barely registers, which isolates the moisture as the bigger drain.
A pan meeting a cold, damp steak is that radiator meeting the damp cloth, hotter and on a larger scale. The drop is normal and expected, and treating it as such changes how the process is managed.
Managing the first few seconds
The response is to prepare for the drop before it happens. Patting the meat dry removes the larger of the two drains before it reaches the pan, and letting it come up from refrigerator temperature trims the conductive one. Preheating the pan well above the searing temperature builds in a margin for the fall that is coming. Crowding several pieces into one pan puts several simultaneous demands on the same store of heat.
The same logic runs across batches. Searing pieces one after another without letting the pan recover leaves each one meeting a pan that starts lower than the last, which is why a rushed batch sears worse as it goes even though the burner never changed. Seasoning, timing and resting all sit downstream of getting that one brief heat transfer contest right, with the pan holding enough heat in reserve to meet something cold and wet and stay above the temperature searing needs.