Like Ellieblue, I use my index finger to move the yarn, rather than the middle finger.
In the Continental purl video, the index finger holds the yarn far up from the needle. I work with my finger much closer, maybe an inch and a half of yarn, no room for the middle finger, lol. I form the stitch by sort of scooping with the right needle as I push/wrap the yarn down and to the right at the same time. My finger actually blocks my view of the stitch for a second as I pull the right needle back out.
I sometimes use my left or right index finger to sort of hold the stitch for a sec as I pull the right needle out the back, but that's usually only at the beginning or end of a row when there's less tension from previous stitches.
Wow, it's really hard to describe, hope it makes sense. In any case, just practice a lot. After a while you'll get a rhythm and way of forming the stitch that's perfect for YOU. Although I watched the videos, after enough practice I just found myself doing it "my" way.