Purl through back loop

I’ve searched and all I can find is different ways to do a purl stitch but not purl through back loop! Can anyone direct me to the video that shows me purl through back loop?

Halfway down the page in the glossary at the tab above there is instructions.
http://www.knittinghelp.com/videos/knitting-glossary

I found a great video on utube: verena knitting how to purl thru the back loop. Just go to utube .com and search this entire topic it will come up. Good luck!