The Special Expanding Symbol is one of the most iconic and popular mechanisms in online slot history, a core free spins feature primarily found in 'Book of...' series games. The essence of this feature is that one regular symbol, randomly selected at the start of the free spins, gains a special power. This symbol, as long as it appears on the reels in the required number to form a win, will expand to cover its entire reel and create massive wins regardless of its position.