Short answer first, then the reasoning — that is how rules questions deserve to be answered, because you usually need this ruling in the mid...
Short answer first, then the reasoning — that is how rules questions deserve to be answered, because you usually need this ruling in the middle of a game.
The answer
Cascade triggers when you cast the spell, before it resolves. You exile cards from the top of your library until you exile a nonland card whose mana value is less than the cascading spell's mana value. You may cast that card without paying its mana cost. Then every other card you exiled for this cascade — including the lands — goes on the bottom of your library in a random order, and the original spell continues resolving. Two details decide real games: the free spell must cost less, not less-or-equal, and casting it is optional — you may decline and simply bottom everything. A second cascade ability on the same spell (some creatures have cascade twice) repeats the whole process for a second free spell, which is why those cards produce such explosive turns.
The cards involved, on their real text
Bloodbraid Elf (mana cost {2}{R}{G}; Creature — Elf Berserker; 3/2; rarity: uncommon; most recent printing listed by Scryfall: Double Masters 2022): Haste (This creature can attack and {T} as soon as it comes under your control.)
Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.)
Maelstrom Wanderer (mana cost {5}{G}{U}{R}; Legendary Creature — Elemental; 7/5; rarity: rare; most recent printing listed by Scryfall: Lorwyn Eclipsed Commander): Creatures you control have haste.
Cascade, cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order. Then do it again.)
Build and check your list with the MTGBrew card database, and browse every card mentioned here in the MTGBrew Learn guides on MTGBrew.
If this topic is on your bench, keep going with Arcane Signet in Commander: Clean Two-Mana Ramp Explained — it covers the neighbouring decisions from a different angle, and the two pieces together give you a fuller picture than either one alone.
Keep reading
Keep reading: Arcane Signet in Commander: Clean Two-Mana Ramp Explained and Kraum, Ludevic's Opus Commander Spotlight: What This Legend Brings to the Command Zone.
Why this keeps coming up
Most rules confusion is really wording confusion: two game words that sound similar — destroy and exile, damage and life loss, target and choose — mean different things, and the difference is deliberate. When a ruling surprises you, read the exact words on the cards involved, in the order a judge would. Nine times out of ten, the text already contains the answer, which is why MTGBrew's guides quote oracle text instead of paraphrasing it from memory.
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