I know the opposite has been posted , but i didn't find a post regarding this so i ll go
ahead and tell you how its done in case somebody needs it , its not rocket science
just simple timing.
1 ) Que for a battleground
2 ) When que pops , join for an instance
3 ) Watch instance queue popping while you are in the battleground
Imageshack - wowscrnshot120610034838.jpg
Notice that while in battleground you won't have the queue icon appear on minimap
so don't get fooled by this.
It can be helpful if you want to leave a losing battleground and you dont want to afk out.
[Edit] I did some further testing and it seems in order for this to work , your battleground queue must bug first. Sometimes the que pops but you can't join and you can't leave que either You have to wait 2 minutes. After that happens , all the above can work. This is for EU servers , dunno about US ones.