What's a decreasing BO?

Finally at the cap part of the sleeve on the Madison by Dolce. The final BO directions after decreasing numerous rows reads: BO using decreasing BO. Not sure what they mean by that. Help please.

Here’s a video that shows you a couple of ways to do it. http://www.youtube.com/watch?v=20BQ1dbP0kI

I’m thinking since it’s an alternating rib pattern that knitting/purling the 2 together may be the best route to go??

Yes, that sounds like a good idea. I was thinking that for all knits you might use half knit 2together through the back loop and half k2tog. So knitting/purling may work well for your pattern. This is really a very nice pattern. I hope we get to see the finished sweater?