According to the commit in question (http://cgit.freedesktop.org/xorg/xserver/commit/?id=7d2543a3cb3089241982ce4f8984fd723d5312a1 http://cgit.freedesktop.org/xo

According to the commit in question (http://cgit.freedesktop.org/xorg/xserver/commit/?id=7d2543a3cb3089241982ce4f8984fd723d5312a1 http://cgit.freedesktop.org/xorg/xserver/commit/?id=7d2543a3...), the feature either releases all grabs or kills clients which have any active grabs. A regular fullscreen application (such as MPlayer, VLC, Chromium, …) does not grab the keyboard and/or pointer. However, applications like pinentry-gtk (for things like entering your GPG passphrase) or screenlockers do grab the keyboard and pointer. So, in short: no, this doesn’t seem useful in that case. And what you are usually intended to do is use the shortcuts of your window manager to kill the window/application or switch to a different workspace/desktop with a shortcut and kill the process from there.