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Reel Expansion is a feature where the physical size of the game grid (reels or rows) actually gets bigger, typically upon entering a bonus round like free spins. This goes beyond simply adding a multiplier; it aims to expand the 'board' of the game itself, providing players with a wider field of opportunity and fundamentally increasing the number of winning ways.

Hold and Respin, or 'Hold & Win', is one of the most popular core bonus game mechanics in modern slots. The essence of this feature, unlike traditional free spins, is to 'Hold' bonus symbols on the reels and 'Respin' the remaining empty spaces to fill up the screen. This system was popularized worldwide through 'Money Train 2's' 'Money Cart Bonus' and is the foundation for numerous developer-specific brand features like Wazdan's 'Hold the Jackpot™' and Playtech's 'Fire Blaze™'.