The video Plantgoddess mentioned shows how to do it.
It is an increase, creating two stitches from one.
You begin to knit the stitch as normal, by pulling yarn though the front leg of a stitch. Instead of slipping the stitch off your left needle as you normally would at this point, you then go and knit into the back leg of the same stitch, creating a second loop on your right needle. Then you can slip the worked stitch off your left needle.