Cascade is a triggered ability that lets you dig through your library for a cheaper nonland card and potentially cast it without paying its mana cost. This turns one spell into two while rewarding careful deckbuilding around mana values.
Bloodbraid Elf and Shardless Agent famously cascade into lower-cost spells (such as those with no mana cost, like Crashing Footfalls), while Maelstrom Wanderer has two instances of cascade and can generate a formidable pile of free cardboard.
