What does export __GL_YIELD="USLEEP"?

[Vrenn]

My kde used to ignore my vsync.settings for some time. There are a lot of bugs and other pages on the internet, for example:
https://bugs.kde.org/show_bug.cgi?id=322060
http://forum.kde.org/viewtopic.php?f=111&t=117147
https://wiki.archlinux.org/index.php/NVIDIA

All explain, that the problem exists on nvidia-drivers and kde sinde 4.11 and they have two solutions.
1) make a /etc/profile.d/kwin.sh with export __GL_YIELD="USLEEP"
2) enable TrippleBuffering in the nvidia-drivers in xorg.conf (Option "TripleBuffer" "True")
if 2) isn't detected by kde but grep -i triple /var/log/Xorg.0.log tells me tripplebuffering is on export KWIN_TRIPLE_BUFFER=1 in /etc/profile.d/kwin.sh may be helpful.

To make it short:
About TrippleBuffering I know it uses more VRAM and might be slower.
Just point 1) works for me, but I don't understand it at all. What does it?

Thanks for any explanation.
_________________
With nice greetings
Vrenn