The only people that still use rulers to measure length of pull are the wing shooters. When they bring the weapon up from the high ready that they favor, they call it mounting, and for those purposes a long length of pull is an asset.

Most people that shoot M4s these days use the stock in, with their elbow down like shown in the picture. It gives you a short package, keeps your elbow from running into things, and squares your body up to the target.

Most people assume that this helps your rifle plate to catch lead, but it also helps the but to stay on your shoulder. If you turn too much it will slide off.

Some guys actually make pads that will help keep the toe of the stock from sliding.

He's using just the toe so that he doesn't have to lean his head down over the rifle to see the sight.

As for the finger on the trigger... that part is just plain wrong.