While using a window manager like FVWM after changing the configuration it is needed to restart the current session to view the change. That is what usually one would do. However there is a possibility to view the changes without leaving the current WM session. It is possible to start a WM in the current WM. Generally this post is taken form (Thomas Adam's article)[https://plus.google.com/+ThomasAdamXteddy/posts/YqzdH3jeAEg] about this. He describes a testing environment using FVWMConsole and the xorg-server Xephyr application.
Start Xephyr:
Xephyr :3 -ac -screen 1024x768 &
Export the terminal:
DISPLAY=:3
Start now the WM inside the same Terminal
fvwm -f ~/.fvwm/config
Et voila. This way you can test a WM configuration without restarting each time your running session.