I see where the misunderstanding is...
12/6 means an 18 hour day... on for 12 off for 6, on for 12 off for 6, 12, 6, 12, 6...
It basically shortens the length of a day for the plant to an 18 hour day. Like I said, I've never tried it and I think it would be a pain in the ass because the lights would come on at a different time each day... I don't even know if you could get a timer to do this... but I've read that it works so I just put it out there (and I also read that shorting the daytime under 12 hours doesn't work - but I don't have any experience with this either).