I find that asking for "mal passado" (literally "badly passed")usually gets me a piece of picanha that is medium rare. Usually I have to ask for "muito mal passado, quase crua" ("very badly passed, almost raw") to get the very rare beef I enjoy best, though, and it always comes with a "crazy gringa" look from the server/carver. rsrsrsrs/hahaha
Brazilians seem to like beef that is (over) cooked to well-done shoe leather, salted of course. Maybe it stems from generally observed poor hygiene standards for meat storage prior to purchase in many places.