Yeah, pretty much what Pathfinder said. Marasi had raw strength, and stronger Mistborn can use their powers unconsciously (Elend and Vin burning pewter while extremely injured and out of it, along with Vin's "luck" are good examples). Marasi had a little experience with the weight metalminds on the ship, so she knew what to do once she realized the Bands were what they were, and Allomancy is enough of a well-documented thing by this point that she has probably read about what the various metals do/how to handle them (as much as you can get that from books) with the rest just the result of raw strength.