Why does this work? (defrosting steak)
Have you ever noticed that if you walk outside on a cold day and touch a metal object, the metal feels colder then the air? The reason is, solids and liquids transfer heat better than gasses do. If you set a steak on a plate to defrost, there is air above it, and a cushion of air below. When you sandwich the steak between two pots, one of which has a large mass of warm water in it, the heat from the water will flow into the meat, and the weight of the water squeezes the air out from beneith the steak, ensuring it makes good contact with the pot below. This all increases heat transfer and the rate of defrosting.

Edit: on a similar note, you can see the “cushion of air below” effect when boiling water. As a pot of water nears a boil, and there are just small bubbles forming and collapsing on the bottom of the pot, pressing down on the handles will improve the solid-solid contact area and you will see (and hear) the rate of bubble formation increase...and then decrease when you release the handles.