When purling through back loop. You will be twisting the YO so there won't be much of a hole like there usually is when doing a YO. To purl tbl, you want to place your needle from the back all the way around the back leg of the yo (left side of the leg, to the front, then purl. You will be almost purling it from the back side to the front. Hope this makes sense. There should be a video on youtube.com if it still is confusing.