This refers to your entire posture and is also referred to as “Run Tall”. You lengthen your neck, but keep your chin level. Don’t let it drift upward. Your knees are “softened” out of a locked position and your core is engaged. You should be able to look down and see the laces of your shoes (assuming a flat tummy).
I found I had been doing two major things wrong. I had been bending my knees way too much. Once you bend them more than even a little bit you engage your quads on every step, which is not good. The other thing was that I needed to do was “unlock” my waist. When I was holding my core I was clenching my back muscles which pulled my chest back, slightly arching my back backward. I needed to add a slight bend forward at the waist to move into the right position. Something I would have never figured out without a coach looking at me. And by “bend” I mean barely.. similarly to “softening” the knees. I find this one of the hardest things to do consistently.
My Disclaimer: I am not a Chi Running instructor or an expert runner or a coaching professional of any kind. These represent my notes, primarily for my own edification. If they are helpful to you then that is great and I am glad for that, but I don’t guarantee that everything here is absolutely accurate as they are a reflection of my own experience and memory.