"p2tog-b"?

Hello. I’m working on a collar for a cardigan. This row reads: K1, p2tog, pat to last 3 sts, p2tog-b, k1. What is the meaning of the “b” in the second p2tog???
Thanks!
:XX:

The b is for you to purl two together through the back loops. It’s a little awkward–you bring your right needle around and insert it from left to right through the back loops and then purl those stitches.

Here’s a video…
http://knittinghelp.com/knitting/videos/decrease/p2tog-tbl-english.mpg