Fixed at/on the poll

If we take the definition of the poll to be this

Definition: The poll is the area immediately behind the ears and the underlying bones are the top of the skull bone and the atlas bones of the neck. In this area are many nerve endings and acupressure points.

I interpret it as tension and lack of flex in this area. If you consider the spine to be continuous from this point right along to the tip of the tail, a horse thats working well will be fluid and relaxed all the way along, as opposed to a horse thats 'fixed at/on the poll'.

Thats my take, anyway :)
 
Equestrians seek collection at the poll.

Technically speaking, the poll is a bony point at the very top of an equine skull, located slightly behind and right between the horse's ears. An equine's poll is situated between the frontal bone and the temporal fossa. When a horse stands erect and tall, the poll is the highest point of his entire skeletal structure.

Among equestrians, however, the term "poll" has gained a slightly broader usage. Most horsey folks discuss the poll as a poll joint, located at the juncture between the top cervical vertebra or atlas joint (C-1 on the horse's spine) and the cranial bones of a horse's skull. A horse's head usually has a slight indentation at this point.

WHY IS THE HORSE'S POLL IMPORTANT TO HORSEBACK RIDING?

The poll joint is particularly sensitive to pressure. This is the spot where a bridle crownpiece passes over the top of a horse's head. This is also the exact site where a leather, web or rope halter will come into contact with a horse's skull. By applying even slight pressure to the tack, a horse handler or rider can significantly influence the horse.
In nearly any equestrian discipline, a major goal of both trainers and riders is to gain the appropriate level of flexion in the horse, particularly at the poll.

Flexion is a compliant and athletic bending of the horse's poll joint, which indicates that the horse is accepting the bit in his mouth and heeding the handler or rider's cues (particularly the bridle, bit, and reins or lunge line).

For example, in a dressage show, the judges wish to see a horse moving forward willingly with his poll as the highest point of his stature, while holding his face in a vertical position at a 90-degree angle to the ground). Both under-flexion and over-flexion are undesirable.
A horse who proceeds fluidly forward with willing flexion and acceptance of the bit and light rein contact is considered to traveling with collection. From the most basic introductory equestrian levels to Grand Prix competitions, this is the goal and the most desirable way of demonstrating an equine's beauty and natural athleticism in a horse show.

Also, in horse show classes, whether English or Western, when horses must stand for the judges' evaluation, either individually or as a group, they are expected to hold themselves squarely and straightly, with their bodies balanced and even, from poll to tail.
Basically, when it comes to winning in the horse show ring, it's all about balance, and the poll's the goal.

In conclusion to your question fixed at the poll is where a horse has not demostrated athleticism ie fixed at the Poll.

SOURCES:
http://allaboutho rses.com
http://equusdreams.com
http://classicaldressage.co.uk
http://usdf.org
 
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