Reminder text is on the regular version of the card, which is attached to the post.

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.

702.61c: Multiple instances of split second on the same spell are redundant.

I'd say the most common "special actions" that are used at instant speed are turning a face-down morphed/manifested/disguised/cloaked permanent face-up.