Do you have a link to the pattern?
It probably means to knit 5 rows and decrease on the 6th and 12th. Then you would decrease every other row after that--not with 6 more rows.
The reason I'm not sure is that most patterns will say 'decrease in the next and every 6th row' if you are supposed to decrease on the first row of the 6th.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry