CLFSWM

This first screenshot is what you have after you have created some frames

You can see the root frame (the root of the roots). The current root which is the root frame in this case. And the current child which is the red bordered frame. In yellow, you have the next children if you press the up key or the previous child if you press the down key.

This second screenshot is what you have after pressing the down key. That is after selecting the previous child.

This last screenshot is what you get when you enter in the current child.

The selected child in the previous screenshot become the current root and is fullscreen maximized.

If you leave the current root, you'll return in the state of the previous screenshot.