What is an expanding special symbol?
The expanding special symbol is a regular symbol that assumes a special function during the free spins round. It acts as both a sort of scatter and expands to cover the entire reel on which it appears. As a result, the special symbol pays out winnings on all active paylines. For this to happen, it must appear in the minimum required number, which typically corresponds to the standard indication in the paytable.

Interesting facts about the bonus feature.
Origin of the special symbol
The first slot that brought this special feature to your screen is the popular Book of Ra slot by Novoline. It was the Austrian game developer who laid the foundation for all subsequent book-themed slots. Since the release of the original, the bonus feature has hardly changed at all. Modern versions of the game concept primarily focus on contemporary slot designs and expanding upon the original feature.

This is how the feature works.
To witness the special feature in action, you'll need to enter the free spins round, which is triggered by at least 3 book symbols. After that, a symbol will be chosen from the eponymous book. The selection can be any of the regular symbols, except for the scatter. Subsequently, the free spins will automatically commence.
Regular line wins can occur at any time and will be paid out accordingly. If the previously chosen special symbol appears in the required minimum number, it expands to the reels on which it appears. All other symbols are grayed out. Now, winnings are paid out on all active paylines. This works, so to speak, according to the scatter principle, regardless of whether the first reel is occupied or there are interruptions.
Modern variants
I'm delighted to share with you some fascinating advancements in the gaming concept with special symbols. Among these are Rise of Merlin and Legacy of Dead by Play'n GO, which have captivated players. In these slot machines, the free spin round follows the same book-inspired pattern. However, during retriggers, you not only receive more free spins but also another expanding special symbol. This means that in the best-case scenario, you can play with a total of 9 special symbols and rake in substantial winnings.