The Adjustment Brush, which works within the Local Adjustments tab, works like a standard brush tool, with adjustable sizes, feathering and opacity. You can paint an adjustment in or out via the Mode pop-up in the Tool Options bar, and adjust the size and feathering of the brush via the bar or with the bracket keys on your keyboard. The Adjustment Brush is especially powerful when used with the Perfect Brush option, represented by its icon at the right of the Tool Options bar.