Cascade and split cards are a fun little combination, both in Modern and in Commander. The reason it can be fun is that when Cascade asks a split card what its CMC is, you get two answers; one for each side. As long as ONE of those answers is ‘low enough’ for that Cascade, you can cast either side. It’s a binary yes/no question: Does this card have a CMC less than the card that triggered the cascade ability? The answer is “yes” even though the card also has a CMC greater than that.
So say you cast Bloodbraid Elf, and the first card you turn over that has a low enough cost is Boom/Bust. Bust doesn’t have CMC 3 or less… but Boom does. So you can cast Boom/Bust, and as is normal for casting a split card, you choose which side to cast as you put it on the stack (and either way, you get to cast it without paying the mana cost). So it’s a perfectly legal (though perhaps a touch mean-spirited) play to cast Bust off Bloodbraid’s cascade.
Today’s Rules Tip written by Trevor Nuñez