Looks cool somehow but i don't have a clue what this does. Please try to explain again. Sorry,

I'm so sorry for my poor English.
When you make a replay start a new thread,You can use vbcode botton to Insert monospace text into your message.It will display like that:After you installed this hack ,you can Insert monospace text into your message too, but you can run and edit or test this code in post.
like this:
http://tsing.com/vb/showthread.php?threadid=1755
It's cool for some webdisign site ,they can use this hack to test javascript or any web page.

I think I get it. This is for sites whose users post html? They can open their message in a new small window and see how the html looks?

yes.
user can see the code in a new window if they press the "run" botton.
but not only html,other codes can be run and test too.