KeyWindow is a tool that change window's layout easily via hot key.
Window management is troublesome in OS X. While using several windows at the same time, we wish these windows can be tiled on the same screen. For example, fill up the left half screen with browser window, and fill up the right half screen with text editor window. To achieve this, we must drag the windows and resize them, inefficiently. Although SplitView comes up since OS X 10.11, it still needs dragging, not convenient enough.
So, what is the most convenient way? Using hot key, of cource. OS X does not support changing window layout with hot key, but KeyWindow bring this feature.