The first judges look for is for a horse that is “willingly guided”.
That means, the horse is soft and extremely responsive, any resistances are negative points, so is anticipating.
That overall picture comes first, then each movement is judged in itself on how close to ideal and how much the rider asks of the horse, the more is asked, the more difficult, the higher the score.
A nice fast spin may be perfect, the horse never putting a foot out of place.
Then, the more speed, the more chances to make a mistake, but if all goes well that is a higher score.
Reining is very technical, mistakes are easy to make.
It is not as easy as it seems when you watch them just cruising along.
Not many riders and horses get to show consistently successfully at the higher levels, it takes talent and well trained skills.
Watching runs in the Futurities, with green horses just learning to show, you can see who has the talent, but also see how easy it is to make all kinds of mistakes.
Once showing at the level here, with older horses, it is more seamless, the differences in scoring more subtle to see, it takes an educated eye to catch why one horse had a better score, hard to see where another one seemed better but scored lower.