Randomly in the base game a random paying symbol is selected to be SPECIAL EXPANDING SYMBOL. When that happens the selected symbol does not explode as part of tumblings and instead, at the end of all tumbles resulted from the initial spin, it expands into a group of four symbols of the same type, in 2×2, after which all winning combinations with it are paid.
By default it expands towards right-down. If the symbol is on the bottom position of reel 1 it expands right-up. If the symbol is on reel 7, anywhere except the bottom position, it expands left-down. If the symbol is on the bottom position of reel 7 it expands left-up.
When expanding different instances of the symbol can expand into the same place and overlap each other. If this happens all instances of the symbol in one place are counted as a single one.
This is the SCATTER symbol.