If a non-Red Chest spawns hearts, coins, bombs and/or keys, causes the chest to spawn a second group of said pickups, effectively doubling the average pickup yield.
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Notes
Mom's Key cannot change the contents of a chest; it will only cause it to spawn extra pickups.
All other chest rewards (such as pills, cards, items, etc.) will not cause the chest to spawn additional pickups.
In-game Footage
Trivia
Mom's Key is the first item, by ID number, that was not carried over from Wrath of the Lamb.
The appearance of Mom's Key is a reference to Bowser's boss key in Super Mario 64 as well as the Boss Key in various Legend of Zelda games.