The brush tool has all the same blend modes as layers, but the brush tool includes two additional blend modes: Behind and Clear.The "Behind" blend mode will apply paint only on transparent pixels in a layer. The "Clear" blend mode turns the pixels you paint on transparent, much like the Eraser tool.
To reset one or all tools to their default state, select any tool, and right mouse-click (Mac: Control click) on the Tool Preset Picker (the tool icon on the top left in the Options bar) to choose to Reset Tool or Reset All Tools.
To activate the mask on the currently selected layer press Ctrl (Mac: Command ) (This is the same as clicking on the layer mask thumbnail). To activate the layer press Ctrl 2 (Mac: Command is the same as clicking on the layer thumbnail.