An X1, X2 or X4 icon may appear in the upper-right corner of the screen.
By tapping the icon, you can turn the Booster on and off. When the Booster is activated, you will spend more energy, but generators will produce higher-level items.
For example, X1 means that the Booster is deactivated and you will spend the standard 1 unit of energy to generate an item. X2 means that 2 units of energy are spent per tap, and the level of generated items is increased by 1. With X4, you spend 4 units of energy per tap, but the level of generated items is increased by 2.
Please note: the Booster does not apply to generators that produce items for free or to chests.