The Sharpen tool increases the contrast between neighboring pixels, which makes affected areas appear "sharper". This tool should be used with care since areas affected by the tool can easily be made unattractive.
If this option is active, all layers will, internally, be combined to on. The Sharpen tool will take into account all layers but will only be applied to the active layer.
Use this value to increase or decreasethe tool’s strength.
Otherwise the Sharpen tool works like any other normal brush. The Sharpen tool’s Attribute Manager contains all brush parameters, as already described here.