>702.61a Split second is a static ability that functions only while the spell with split second is on the stack. “Split second” means “As long as this spell is on the stack, players can’t cast other spells or activate abilities that aren’t mana abilities.”
>702.61b Players may activate mana abilities and take special actions while a spell with split second is on the stack. Triggered abilities trigger and are put on the stack as normal while a spell with split second is on the stack.
If you are a cool aristocat I'm here to inform you that Phyrexian Altar and Ashnod's Altar can sac a whole lotta things and stack a whole lotta triggers on top of a split second spell that nobody can respond to