Purl through back loop

I think I know how to P tbl – I just insert my needle through the back of the loop instead of the front, right? But anybody got an idea why a pattern would tell me to do that? Like, does it twist the stitch or what? This is at the end of a row in the brim of a baseball-style cap that will get felted.

Purling through the back loop does twist the stitch to make it a firmer edge. You do it from left to right around the back of your left needle through the back of the loop.

Amy has video.