Siege Smash

Split second is the entire pitch. A modal instant that either blows up an artifact or hands out a combat pump is nothing special on paper; both effects have lived in cheaper, less restrictive shells for a long time. The wrinkle is what the mechanic does to the stack: cast this and no one can respond with a spell or a non-mana activated ability, so most sacrifice outlets can't save the artifact, no protection instant fizzles the pump, no fetchland crack or Equipment shuffle wriggles out from under it. The artifact mode functions as near-unanswerable removal against a combo piece or a Sword that would normally beg for a response. The pump mode is the sleeper: a +3/+2 and trample the defending player cannot answer while the spell resolves, though once it does they regain the chance to respond before damage. Note the exact edge, because it is narrower than it looks: split second locks out spells and non-mana activated abilities, not triggers. A death-trigger, a blocker-generating trigger, anything that fires on its own still goes on the stack and resolves normally. What you are buying is immunity to reactive plays that require casting or activating something, not immunity to the battlefield's automatic responses. Both modes are individually mediocre, and the card is priced to keep them that way; split second is a tax paid in rate and flexibility. It is a scalpel for opponents who rely on responses, and dead weight against anyone who does not.