The short answer is… it doesn’t. When you cast a Bloodbraid Elf, for example, it’s true that you exile cards from the top of the library until you find one that satisfies the condition of being a nonland with lower CMC (let’s use the big bad beater himself, Tarmogoyf). However, even though Goyf is in the exile zone at this time, it’s still cast from that zone (albeit without paying its mana cost). Containment Priest doesn’t care whether a card was cast from the hand or from some other zone, and it doesn’t care if it was cast for its mana cost or some alternative cost. This means that Goyf will enter the battlefield, followed by the original Bloodbraid. If you want to stop cascade, try a counterspell or Stifle instead!
Today’s Rules Tip written by Jen Wong