I love this counterpart to the regular long tail cast-on. It's so simple to work them in combination and cast on anything in ribbing! The long tail cast on actually casts on and makes the first row of knit stitches (which is why you purl the first row when starting in St st.) To start with a seamless 2 x 2 rib, for example, you would cast on 2 sts in the usual method, then 2 in this purl variation, then 2 in regular knit, etc. I used this technique in my Zombies Need Not Apply pulse warmers in the April '06 issue of Magknits. It works particularly well there where the cast on and bind off rows are so close to each other, thus more noticeable if different.
This technique is also perfect for casting on for double knit fabrics. Alternating between knit and purl sets up the stitches so every other one is facing in the opposite direction.
I had more success in illustrating this with a simplified drawing than photographs. With so much going on visually I decided to indicate the movement of the needle with a green arrow.


To make a purl cast on stitch start by holding the yarn in the usual manner. Pick up the strand of yarn on the index finger farthest away from you by coming from behind it.


Move the needle towards the strand on the far side of the thumb and go under it coming from behind to catch it on the needle.


Pull the needle back towards the index finger pulling the yarn through the loop around the index finger. Slip loop off index finger and tighten the stitch.