Expanding wilds is a type of Wild symbol which expands over an entire reel, either upwards or downwards, transforming all of the symbols on that specific reel into a Wild symbol. They appear randomly during a spin so there is no way you can control when these will appear, and in some slot games they can only appear on certain reels and not on all of them. Once they create a winning combination this is usually combined with nice animations, sound effects and music to cheer you up. How big winning you get depends on how many paylines you had activated and your betting amount. Overall, the expanding wild symbols can help you create big winnings which is always welcomed whilst playing.