The Override Tool lets you increase productivity by rapidly switching between tools used for short tasks and your previous tool. Most drawing tool shortcuts are accessed using the [Alt] key followed by another key, such as the Eraser tool which is accessed by pressing [Alt] + [E].
If you are drawing with the Brush tool and need to briefly switch to the Eraser before continuing, hold down the [E] key while you are erasing. Once done, release [E] to return to the previous tool, in this case, the Brush tool. You can do the same for most drawing tools that have a shortcut composed of [Alt] followed by another key.