Split second

Split second is a static ability that does not allow any players to put any spell or activated ability on the stack as long as the spell with Split second is on the stack.

History
Split second was introduced in Time Spiral. Two cards with split second appeared in Planar Chaos and three in Future Sight. It reappeared on one card in Commander 2019.

A historical note is that, in a way, Split second harkens back to interrupts, which also couldn't be responded to with instants or activated abilities, only other interrupts. Split second (originally called "superfast") was originally designed by Devin Low as an Izzet mechanic for Guildpact.

Rulings

 * Players still get priority while a card with split second is on the stack.
 * Split second doesn't prevent players from activating mana abilities.
 * Split second doesn't prevent triggered abilities from triggering. If one does, its controller puts it on the stack and, if applicable, chooses targets for it. Those abilities will resolve as normal.
 * Split second doesn't prevent players from performing special actions. Most notably, players may turn face-down creatures face-up while a spell with split second is on the stack. For example, if Sudden Death is targeting a creature you control, you may flip up a Willbender to change the target of Sudden Death to another creature.
 * Split second does not prevent the spell from being countered by triggered abilities. One such example is Voidmage Apprentice. Decree of Silence can also counter cards with split second.
 * Split second won't affect spells and abilities that are already on the stack.
 * If the resolution of a triggered ability involves casting a spell, that part of the effect won't work if a spell with split second is on the stack.