I thought "pinning" the elbow just meant keeping it totally still during the stroke, as if your lower arm were just a pendulum swinging from a pin. That may not be how most people use the term, though.
Efren keeps his elbow in close and behind, I think it's better on the shoulder because it keeps tension in the right directions to hold the shoulder, shoulder blade and collarbone together. It feels like it shortens my stroke though it doesn't seem to shorten his.
Andrew Manning has the correct definition. The elbow, for the most part, remains motionless, during the backswing and forward swing of the forearm. The grip hand "finishes" in a natural position, close to your armpit. There is little or no tension in the bicep.
Sam...we worked with you on this, at poolschool in Charlotte.