If you use the button on bottom right (Win 7) or Win+D, you "show desktop", basically minimize (hide if minimizing is not possible) every window. What happens with DisplayFusion though is that if you open(i.e. unminimize) again one of those minimized windows on the primary monitor, and you had a window(s) that minimized on secondary monitor, the window(s) reopen too. This happens only with DisplayFusion on. And it's only the main monitor that opens windows on the secondary one, not inverse.