The third pictures shows it the best. It's the curve in his neck about 6" from his ears. It is one component of getting a horse collected.
Yes, thank you. The third picture is definitely the better of the two. In the first, he's bracing a bit. In the third he's easing off of the bridle a bit more because I'm asking for bend in his body as well. (he had been dropping across the arena to avoid the muddy spot there)